CHEMICAL NEWS, March 15, 1862. } Reaction of Ethyl Bases. 143 the last year and a-half in carbonic acid determinations on the surface of the sulphuric acid, will force with such good results as to induce me to bring it under the notice of chemists, in the hope that it may prove as useful to others as it has been to me. Since the introduction of Fresenius and Will's excellent and well-known arrangement for the indirect estimation of carbonic acid, numerous modifications have been proposed, but Fresenius himself, in speaking of these, says: "I have not as yet been able to convince myself that any one of the modifications of our apparatus affords essential advantages." I do not pretend to say that, in general, my arrangement possesses" essential advantages" over that of the distinguished German chemists; but I have no hesitation in saying that, under certain circumstances, it will be found more convenient and equally accurate in its results. Moreover, I have not seen an arrangement similar to this one described in any of the books or periodicals with which I am acquainted, though the principle is so obvious that I can hardly conceive it possible that a similar proposition to mine has not been made before. Description of Apparatus.-The annexed wood-cut will aid me in describing the arrangement which I use. A flask of the required size is taken and fitted with a perfectly sound cork, which is then bored so as to carry the two tubes shown in the cut. The next step is to prepare the little internal tube, which may be easily blown from a piece of moderately stout tubing,care being taken, however, to expand the body of the tube to such an extent as to admit of its only just entering the wide mouth of the flask, at the same time leaving the bottom in the shape of a rather acute cone. The tube, so prepared, should then be fitted with a cork, which must be doubly perforated, in order to receive the tubes shown in the wood-cut, the larger of the two tub s passing through the cork of the flask, thus establishing a communication between the external atmosphere and that within the small tube, at the same time serving as a means of suspension for the latter; the smaller of the two is formed from a piece of quill tubing which should be curved at one extremity, the other reaching to the bottom of the small tube flask, into the cone of which it should closely fit, without actually touching the glass on either side. All that now remains to be done is to fit the piece of quill tubing (closed at one end with a little wax or gutta-percha) into the cork of the larger flask, when the apparatus is complete. Method of using the Apparatus.-In the analysis of the carbonates of bases which form soluble salts with sulphuric acid, the following method is adopted :From ten to twenty grains of the carbonate to be analysed is introduced into the larger flask, together with a little water; the small tube-flask is next about two-thirds filled with concentrated sulphuric acid; the corks are then properly adjusted, and the apparatus wiped perfectly dry and then weighed. After the weight has been carefully ascertained, connect the end of the large tube with a chloride of calcium tube, and apply the lips to the open extremity of the latter, then force air gently into the apparatus, which, by pressing it out through the curved end of the quill tube. Some care is required in this part of the manipulation, as, if the drop of acid be allowed to fall immediately into the solution of the carbonate, too rapid evolution of gas would take place, but if the flask be so inclined that the drop may fall on the external curved surface of the little tube containing the sulphuric acid, the rate at which the gas is evolved is perfectly under control. The moment the gas is liberated it forces its way through the curved tube, and likewise through the sulphuric acid, in bubbling through which it is completely dried, this desirable object being facilitated by the way in which the bubble is given off from the end of the tube, as, having to strike against the conical bottom, it is thus made to present a larger surface to the sulphuric acid; the gas finally escapes through the wide tube into the atmosphere. The manipulation is thus continued, blowing over the acid guttatim until the carbonate is completely decomposed; when this is effected, a small additional quantity of acid is blown over, and the apparatus placed on the sand-bath, or in boiling water, until the temperature is sufficiently raised to expel all the gas held in solution by the water. The wax plug is now taken out of the piece of quill tubing carried by the cork of the flask, and suction applied at the wider tube, until the air ceases to taste acid. The apparatus is now allowed to cool, then wiped perfectly dry, and re-weighed; the loss in weight will indicate the amount of carbonic acid contained in a given weight of the sample under examination. If it is desired to analyse the carbonate of a base which forms an insoluble salt with sulphuric acid, it is only necessary to replace the short tube plugged with wax by a bulb tube containing nitric acid; the latter is held in situ by atmospheric pressure, the upper part of the tube being stopped with wax. When the plug is removed, the acid flows out and decomposes the carbonate; the carbonic acid evolved has still to bubble through the sulphuric acid before escaping into the atmosphere. The remaining steps of the manipulation are precisely the same us in the former case. The apparatus, if well made, and the proportions be properly adjusted, need not weigh more than from five to six hundred grains; though I have an arrangement at present which, when charged with the carbonate, acid, and water, weighs only about 470 grains. For the accuracy of the results obtained by means of this apparatus I can vouch, as I have controlled them repeatedly by the analyses of perfectly pure carbonates of known composition, and my opinion has been confirmed by the testimony of both Professor Cameron and Mr. Tichbourne, Chemist to the Apothecaries' Hall of Ireland, who have carefully ascertained the capabilities of the apparatus before using it generally in their analyses. Dublin. Reaction of Ethyl Bases with Dr. Knop's New HydroAuosilicic Acid, 2HF1+ Si2Fl3, by M. CAREY LEA, Philadelphia. IN the Chemisches Centralblatt for August 21, 1861, Dr. Knop publishes an account of a very interesting combina tion which he has obtained by the action of peroxide of copper and metallic copper on silicofluoric alcohol, and subsequent removal of the copper by sulphydric acid. He thereby obtains an acid 2HFI + SiFl, which exhibits different properties from ordinary hydrofluosilicic acid 3HF1 + 2SiFl3, and which he thinks may prove a valuable re-agent. It precipitates, according to its discoverer, potash and soda completely from their solutions, while with sulphate, chlorhydrate, phosphate, and oxalate of ammonia, it gives no precipitate. It appeared to me to be of interest to examine the behaviour of this re-agent with some of the ethyl bases. I prepared some by Dr. Knop's process; with it obtained immediate precipitates with carbonated and caustic alkalies, hydrochlorate of ammonia afforded no precipitate-results in accordance with his. Caustic ammonia afforded the following results: When the acid was in excess no precipitate was formed, but where the ammonia was in excess a very abundant caseous precipitate was obtained, which showed little disposition to dissolve in an excess of precipitant, even by standing and with the application of heat. Chlorhydric acid dissolved it somewhat better, but still left a considerable portion undissolved. Ethylamine, when the acid was in excess did not give a precipitate, but when the ethylamine was in excess, the mixture, by standing awhile, coagulated to a jelly so stiff that the test-glass could be inverted, and this with scarcely any loss of transparency. In an excess of the new acid, this jelly re-dissolved, with the exception of a few flakes. Diethylamine exhibited almost the same reaction as ethylamine, except that the jelly was less transparent, and the solution in excess of the re-agent less complete. The acid gives no precipitate with either the chlorhydrate of ethylamine or that of diethylamine. In this respect, therefore, these ethyl bases resemble ammonia. -American Journal of Science. On Crystallised Platinum, by Dr. T. L. PHIPSON, F.C.S., &c. WHEN a plate of platinum is left for about two months in a mixture of nitric and hydrochloric acids in a moderately warm place, the surface of the plate has, at the end of that time, become perfectly crystalline, whilst a certain quantity of the metal has been dissolved, On examining the surface with a proper magnifying power, it is seen covered with an infinite number of crystalline scales, some of which lie flat upon the surface, others being elevated above it; they all appear to be of the tetrahedron or octahedron form. The only mention I find of crystallised platinum is in Berzelius (Traité de Chim.), who says that some of the small grains of native platinum occasionally shows traces of crystallisation. 4. Potash and Amylic Alcohol. observation, only the absorption is slower than with the other two alcohols. The difference rises to about onehalf. It is perhaps due to the viscosity of amy lic alcohol. 5. Potash and Glycerine. The absorption is much slower than with water, which is doubtless due to the viscosity. 6. Potash and Ordinary Ether.—The absorption is more rapid than with any other substance. 7. Potash and Compound Ethers.—Acetic ether is without any marked influence; with nitric ether the absorption is more marked than with water, which is no doubt due to the formation of ordinary ether. 8. Soda and Alcohol.—Soda behaves like potash. 9. Lime and Alcohol.-The absorption does not appear more rapid than in the presence of aqueous potash. 10. Baryta and Alcohol.-Alcohol hastens the absorption of carbonic oxide by baryta; this absorption is then very marked. 11. Baryta and Wood-spirit.—The influence of wood-spirit is the same as that of alcohol. 12. Baryta and Ether.—The absorption is at first hastened if the ether contains water; it afterwards stops, owing to the absence of the water necessary for the reaction. An alcoholic solution of ammonia does not absorb carbonic oxide at the ordinary temperature.-Annales de Chimie et de Physique, lxi., 463. On the Quantitative Determination of Lead in Chemical Analysis, by M. A. LEVOL. THE precise estimation of lead, though presenting no serious difficulties, nevertheless demands precautions sufficiently minute. The object of this paper is to state the results of my own observations on this subject. The Quantitative Determination of Lead in the State of Sulphate. The estimation of lead in the state of sulphate, by means of sulphuric acid and evapo. rating to dryness, insures accuracy, but the process requires constant attention. Towards the end of the analysis the evaporation exposes it to loss by projection; moreover, if the liquids contain iron, the sulphate of lead is often contaminated with the slightly soluble ferric sulphate. The solubility of sulphate of lead, even in water, is well known, as the following experiment shows:-Precipitate one equivalent of nitrate of lead by water; then wash during several days, and long after two equivalents of sulphuric acid diluted largely with the washings have ceased to redden litmus-paper, they will still become slightly turbid by nitrate of baryta and hydrosulphate of ammonia. The use of soluble sulphates, suggested by various authors, is not to be recommended, as will be shown. My opinion was, that the principal inconvenience arose from the incomplete insolubility of sulphate of lead, and that, consequently the employment of alkaline sulphates would produce but imperfect results. I was CHEMICAL NEWS, March 15, 1862. Generalisation of the Acidimetric Method. then much surprised to find, under such circumstances that the fact could not escape notice, an overweight, in precipitating lead by sulphate of potash. If, in fact, liquids much charged with nitrate of lead and sulphate of potash or soda in excess are put in contact, precipitates are obtained, the weight of which considerably exceeds that of the sulphate of lead, corresponding to the weight of nitrate, and it is with difficulty that they are reduced to this weight by washing. I will state some of the results:-51775 grammes (= 1 equivalent) of nitrate of lead, added to 5'455 grammes (= 2 equivalents) of sulphate of potash, yielded a precipitate which simply drained, then dried at 110° and weighed on the filter yielded 7.355 grammes, instead of 4.739 grammes. This precipitate was fusible, and to separate from it the sulphate it contained, no less than ninety washings in cold water were required. An experiment made with nitrate of lead and sulphate of soda yielded 5'325 grammes of drained precipitate, instead of 4739 grammes; and to extract from it the sulphate of soda thirty-six washings in cold water were grammes. = After the prolonged washings which I have mentioned, the sulphate of lead precipitated by sulphate of soda weighed 4 640 grammes, the loss by washing having been o'099 gramme. The previous precipitate = 4'585 grammes. Loss by washings =0*154 19 Independently of less stability, this difference seems to indicate greater solubility of the double sulphate of lead and soda, which is further rendered apparent by testing the washings by means of baryta and hydrosulphate of ammonia (apart from the solubility proper to the sulphate of lead, which I have taken into account in this instance), by comparing with it a third washing of sulphate of lead formed by sulphuric acid and nitrate of lead, equivalent to equivalent. It appears, then, I repeat, that there are formed by the wet way, under certain conditions, double sulphates of lead and potash, or soda. I have found no similar salts mentioned in any French work; but in a paper, published in 1825, by Tromsdorff, it is pointed out that the potassic salt is obtainable by the precipitation of acetate of lead by sulphate of potash. The author adds, that by boiling this salt with a large quantity of water, the proportion of sulphate of potash which it contains gradually diminishes. It is only under particular conditions of concentration of the liquids that these salts can be formed, and I am certain that the estimation of sulphates by means of standard nitrate of lead, which I have pointed out (Bulletin de la Société d'Encouragement, 1853), are not in any way affected by it. The mode of estimation would be out of place here. On the whole, then, experience shows that alkaline sulphates should not be applied to the estimation of lead in the state of sulphate, by weighing the precipitate, partly because of the danger about to be described, and partly because of the fear of loss of sulphate of lead, by 145 the numerous washings necessitated by the decomposition of the double salts by water. Determination of Lead in the State of Car bonate.-In face of the difficulties to be encountered in estimating lead with great precision, it seems to me highly to be recommended that it should be determined in the state of carbonate, if for that purpose ordinary carbonate of ammonia, to which is added caustic ammonia, is used. The object of this addition is to avoid the employment of too large a volume of solution of carbonate of ammonia, a salt not very soluble in water. The ammonia forms, with nitrate of lead, for instance, a very incomplete precipitate, the composition of which is represented by 2(6PbO.NO) +3 HỒ. It would not, then, be prudent, to divide the operation into two,- that is to say, by employing ammonia first to saturate the liquid, and, consequently it should not be poured in until it has been charged with carbonate of ammonia, which it dissolves abundantly and easily. The precipitate separates perfectly from the liquid, is easily collected and dried on a filter. The deposition of the precipitate is completed in about twenty-four hours, especially under the influence of gentle heat. According to my experience two or three thousandths of lead can be estimated by this process. The precipitate, which is anhydrous, PbO.CO2, is deposited on a small double filter, each one of the same weight. If, as frequently happens, in analysing metallic substances, the colour, which should be pure white, is yellowish, it is owing to the presence of iron, which is easily got rid of by washing the filter with water acidulated with sulphuric acid after weighing. If there is reason to suspect the presence of bismuth, treat a small quantity of the weighed precipitate by a little nitric acid. A few drops of iodide of potassium in the liquid will detect the presence of bismuth by the forming of a brown precipitate, or yellow-brown if there is bismuth and lead The latter metal, when present alone, gives a pure yellow precipitate. Determination of Lead by Oxalic Acid.-I have stated that, in estimating lead by carbonate of ammonia, in presence of an excess of ammonia, two or three thousandths of this metal can be determined. By operating, under the same conditions, with oxalic acid, I have found it impossible to determine it to less than 1 per cent. Writers have, in truth, observed, that the precipitation of lead by oxalic acid should be effected in neutral liquids; but I would remark that this necessity but ill agrees with the most ordinary instances of the analysis of metallic substances, where the presence of an excess of ammonia is indispensable for maintaining in solution certain substances from which the lead should be separated.-Répertoire de Chimie Appliquée, iv. 21. Generalisation of the Acidimetric Method, by MM. E. LANGER and R. WAWINKIEWICZ. M. BUNSEN has proposed an acidimetric method which admits of estimating by standard solutions acids combined with the bases which are completely precipitated by caustic or carbonated alkalies. This method consists in adding an excess of the alkaline standard liquid to the solution of the salt, separating the precipitate by filtration, and determining the excess of alkali in the liquid by means of an acid liquid of a known standard. The authors have assayed the application of this process in a great number of salts; the results obtained are in general very close to those required by theory. The standard solutions employed were caustic potash, carbonate of soda, and hydrochloric acid.-Annalen der Chemie und Pharmacie, cxvii., 230. On Crystallised Manganate of Soda, A CRUCIBLE is filled with a mixture of equal parts of NaO, MnO3 + 10 HO. When dissolved in water, they partially decompose, forming a green solution. The crystals can only be dried on bricks or porous porcelain; they immediately decompose in the presence of organic matter.-Journal für Praktische Chemie, lxxxii., 58. TECHNICAL CHEMISTRY. Chemical and Physical Modifications of the Atmosphere Consequent on Habitation. THE repeated observations of chemists have taught us to regard the identity of composition of the atmosphere as a fixed law-one to which no exception is to be found in Nature, unless it be in the neighbourhood of tropical rivers, where vast quantities of organic matter, the débris of a luxuriant vegitation, are rapidly passing into decomposition. Everywhere, whether collected on the top of Mont Blanc, or on the banks of the Seine or Thames, or in the middle of the Atlantic, the two main constituents of the atmosphere are found in precisely the same proportion, and the more perfect the processes of analysis have become, the firmer has the constancy of this relation been established. This fact has always, however, been rebelled against by the common experience of mankind; it has been almost an opprobrium to science that, in spite of the manifestly different feeling of the air on the Swiss mountain, and in the middle of London, the chemist can detect no difference in composition. During the last few years several chemists have directed their attention to this apparent inconsistency between the organoleptic and physical characters of the air with special reference to the condition of the atmosphere in towns. These researches have related mainly to the quantity of carbonic acid, and other products of combustion, and to the existence of organic matter in suspension. Among the most important are those of Dr. Dundas Thomson and Dr. Angus Smith. The per-centage of carbonic acid usually existing in the air of London was found by Dr. Roscoe3, to be 0037 per volume, a result not differing materially from those obtained by Dumas and Boussingault in Paris. The 1 Daniell: Philosophical Magazine, Fifth Series, No. 121. 2 Dumas and Boussingault: "Récherches sur la véritable Constitution de l'Air;" Ann. de Chimie et de Physique, troisième série, tome iii., p. 257. 1851. Roscoe: "On the Atmosphere of Dwelling-bouses;" Quarterly Journal of the Chemic Society, October, 1857, p. 259. analyses on which these are based were made by passing a known volume of air over weighed tubes containing alternately pumice-stone steeped in sulphuric acid and potash, a method which leaves nothing to be desired in respect of accuracy. Dr. Smith's estimations of the carbonic acid of the air of Manchester, made by the same method, gives somewhat higher results. He found that on a windy day they averaged 0.045 to from 0.08 per cent., and that on a still day the per centage amounted to 0.12. When, however, we consider that although London is the greatest city in the world, Manchester is the largest manufacturing town, and that it is the centre of a manufacturing district comprehending many hundred square miles, over which an atmosphere darkened by smoke perpetually hangs, we are not surprised to find that the products of combustion exist in larger proportion than in London or Paris. Dr. Smith has calculated from the quantity of coal burnt in the neighbourhood of Manchester, that 15,000 tons of carbonic acid must be introduced into the atmosphere daily, without taking into account the quantity expired by man and animals. A much more important product of combustion is derived from the oxidation of the sulphur contained in coal, and the introduction thereby into the atmosphere of sulphurous and sulphuric acids. In the researches undertaken by Dr. Thomsons during the last epidemic of cholera, which consisted in passing large quantities of the air of London through distilled water, it was found that such air invariably possessed an acid reaction, and that this reaction was due to sulphuric acid. Dr. Smith has further investigated this question, and has found that in Manchester the acid reaction of the atmosphere is much more constant and intense than in London. Blue litmus paper becomes red in half an hour, and sometimes in ten minutes when exposed to Manchester rain, and occasionally its acidity is such that a single drop is sufficient to effect the reaction. quantity, however, is exceedingly small; of a solution containing a thousandth part of its weight of carbonate of soda, quantities varying from ten to fifty grains suffice to neutralize 1000 grains of such rain; and as much cistern-water is found to be neutralised by twenty-five grains; from which results Dr. Smith concludes that the largest quantity of sulphur acids existing in the atmosphere of the town does not exceed o·004 per cent. by weight, a proportion amounting to not more than a twentieth part of that of the carbonic acid. As to the The actual The researches of Hlasiwetz in 1856 (Berichte der Wiener Akad. d. Wissens., vol. xx., p. 189) have shown that the sulphuric acid used for the absorption of the moisture, on the one hand, absorbs considerable of the carbonic acid from the air passed over it, which circumstance causes considerable variation in the results. On the other hand, the caustic alkalies which are employed for the absorption of carbonic acid, absorb considerable quantities of atmospheric oxygen. One volumetric analysis of carbonates, which give results in every respect should suppose that the in provements made of late years in the more accurate and expeditious than those obtained by weighing small quantities of gases in large apparatus; would deserve more extended application in the analysis of the air-Dr. Hugo von Gilm, under the guidance of Hlasiwetz (Berichte d. Wien. Akad., vol. xxiv., p. 279), and D. Pettenkoter have contrived methods which leave nothing to be desired in this respect. It was also noticed, that when ozonised air was conducted through weighed tubes containing chloride of calcium, ozone displayed an equivalent proportion of chlorine, which being four and a half times heavier than the ozone, must necessarily cause very great variations in the result. This error is likewise avoided by a previous volumetric determination or absorption of the active oxygen. A further source of error in the determination of the amount of water in the air by means of sulphuric acid is the aumonia present. From all of this it will be seen that the examination of air by passing it through a series of weighed tubes containing alternately various absorbents can only give results which, to say the least, are very doubtful. Appendix to "Report of the Committee for Scientific Inquiries in Relation to the Cholera Epidemic of 1854," p. 110. OBEMICAL NEWS, Chemical and Physical Modifications of the Atmosphere. share of sulphurous and sulphuric acids respectively in this total, it is of course impossible to arrive at a conclusion; but considering what we know of the rapidity with which the former is oxidised in the air, it is to be supposed that whenever the acidity of the atmosphere is marked, it will be mainly owing to the latter. The impregnation of the rain with the mineral acid must be regarded as rather beneficial to health than otherwise, as tending to retard the putrefaction of animal matter on which it falls. Dr. Dundas Thomson appears to have been among the first to recognise the importance of organic matter as a constituent of the air of towns, and to express the conviction that the gaseous products evolved during putrefaction are not the main sources of danger. Proceeding on this idea, he subjected a large quantity of atmospheric air to chemical investigation, "with a view of condensing any vapour, or detaining solid particles, which might be disseminated." The result was entirely negative. Further inquiries of the same kind were made, under the sanction of the Board of Health, in 1854, the air being passed, as has already been mentioned, through distilled water, the result invariably being that hyphaceous fungi made their appearance in the water, and in a short time, by their rapid growth, pervaded the whole of it, so as to be evident to the unassisted eye. It was also found, that on passing the air through sulphuric acid in the same manner, the acid became soon dark coloured, in consequence of the charring of the organic matter introduced into it. Dr. A. Smith has worked out the idea much more completely. He has preferred a chemical to a microscopical test for the detection of the suspended organic matter. It consists, as most of our readers may be aware, in passing the air through a very dilute solution of permanganate of potash, the strength of which is determined by ascertaining how much is required to decompose a solution of a weighed quantity of oxalic acid, or of uncrystallisable sugar. This test obviously indicates, not the quantity of organic matter, but the quantity of oxidisable matter in the atmosphere, and hence it is only valuable in so far as we may assume that the atmosphere contains no reducing agent. Thus, in the presence of sulphurous acid, it would be of little value were it not that the agent exists, even in the town air, in exceedingly small quantity. Many of Dr. Smith's results are of such a nature as to be beyond the possible limits of this source of error. It was found that the same quantity of the salution of permanganate which was decolorised by one bottle of air obtained in a close court in Manchester, required twenty-two bottles to decolourise it on the hills in the neighbourhood. Assuming that sugar and the organic matter of the air are decomposed by the same amount of manganate, "a supposition which cannot be perfectly true, but which, from the minuteness of the amounts, leaves no room for a great error," Dr. Smith concludes that whereas, on the high ground north of Manchester, there existed but one grain of organic matter in 200,000 cubic inches, in close places in the town there was a grain in 8000 cubic inches. From his most recent observations he concludes that we have, "in different air breathed by people in the same country, a substance the amount of which in one case is twenty-two times greater than in the other, and in air breathed by people in the same town a difference which is as 9 to 22." The whole importance of these investigations, reChemical Researches on the Nature and Causes of Cholera;" Medico-Chirurgical Transactions, vol. xiii. 1859. 7 "Report on Cholera," p. 127. 147 garded from the point of view of preventive medicine, lies in their relation to the putrefactive process. To discover a means of seizing upon and estimating putrid exhalations-under which term we include everything not gaseous that is disengaged into the atmosphere from the surface of living animals, no less than the exhalations from dead animal matter-would be certainly a most important step towards acquiring a more satisfactory knowledge of the influence of habitation on health. We have, therefore, to inquire what grounds there are for regarding Dr. Smith's test, or any other founded on a similar reaction, as affording a solution of this problem. It is not difficult to satisfy ourselves that animal matter in putrefaction does disengage from its surface portions of its substance, of sufficient tenuity to be suspended in the atmosphere. Without referring to offensive smells, which of course must be material, we have several satisfactory proofs. If a bell-glass be inverted over decomposing animal matter in a moist condition, the inner surface of the glass becomes in a few days bedewed with moisture, which on being, examined under the microscope is found to contain the same filamentous fungi to which reference has already been made; and on evaporation it leaves a residue, which is blackened by incineration. Similarly we find that the moisture which is deposited in glutinous drops on the sides and arched roofs of sewers, is rich in organic matter, which must clearly have been derived from the air of the sewer. Dr. Smith has related the results of experiments showing that air kept for a length of time in contact with putrescent matter, becomes loaded with oxidizable material, and acquires the power of decomposing a correspondingly large quantity of permanganate of potash. Another group of facts shows us that the existence of putrescent impurity in the air is a principal, though not a necessary, condition of the induction of putrefaction in bodies susceptible of the change. Thus, for example, I have found that milk which retained its freshness for hours, will at once turn on being exposed to a putrid emanation. Butchers are familiar with the fact, that meat cannot be successfully dressed in the neighbourhood of a stinking gully-grate, or of a stable reeking with ammonia; and for the same reason, every intelligent butcher keeps his slaughter-house in a state of scrupulous cleanliness. It is not, however, to be forgotten that other causes, possibly electrical, the nature of which is still involved in obscurity, have a still greater influence in inducing putrefaction. Thus, in this country, the butcher finds that on one day he is able to slaughter and dress even veal or lamb with safety; whereas on another, not differing in temperature, incipient putrefaction may render the carcass unsaleable, in spite of the most careful precautions; butchers are apt to believe that this occurs mostly on calm days when the air feels heavy. Still more remarkable are the facts recorded respecting the slaughtering of cattle in hot countries; the operation can only be safely performed when the air is clear, and the sky cloudless. Under such circumstances, we are told that the appearance on the distant horizon of a cloud "like a man's hand," the sure precursor of a storm, is a sign to the slaughterers on the Pampas of South America to desist from their work, for it is immediately followed by rapid putrefaction. Air contaminated with putrescent matter is for the most part alkaline. Thus the air of sewers is invariably so, as has been proved by the experiments of Dr. Dundas Thomson, its alkalinity being owing partly to ammonia, partly to the sulphuret of ammonium, the form |