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which both depend is a taste for fuperfluity in those who have an equivalent to give ; this taste is what produces demand, and this again is the main spring of the whole operation.

• CHAP. II. We have substituted throughout this book, the term demand, to express the idea we conveyed in the last by that of wants ; and since the subject becomes more complex, and that we have many more relations to take in, I must make a recapitulation of all the different acceptations of this term demand.

Demand, in the first place, is always relative to merchandize ; it is the buyer who demands; the seller offers to fale. said to be reciprocal, when there is a double operation, that is, when the seller in the first, becomes the buyer in the second case ; and then, taking the two operations in one view, we call those demanders who have paid the highest price. 3. Demand is simple or compound; fimple, when there is no competition among the buyers ; compound, when there is. 4. It is great or small, according to the quantity demanded. And 5. High or low, according to the price offered. The nature of a gradual encrease of demand, is to encourage industry by augmenting the supply ; that of a sudden encrease, is to make prices rise. This principle has not every where the same efficacy in producing these varieties : it is checked in its operations between merchants, who seek their profit ; and it is accelerated among private people, who seek for subsistence, necessaries, or luxurious gratifications.

« CHAP. III. I come ne to deduce the origin of trade and industry, which I discover from the principles of the first book, where bartering of necessaries was understood to be trade and I find that the progress of this is owing to the progress of multiplication and agriculture. When a people arrive at a moral impossibility of increasing in numbers, there is a stop put to the progress of barter. This grows into trade, by the introduction of a new want (money) which is the universal object of desire to all men.

While the desires of man are regulated by their phyfical wants, they are circumscribed within certain limits. So Soon as they form to themselves others of a political nature, then all bounds are broken down. The difficulty of adapting wants to wants, naturally introduces money, which is an adequate equivalent for every thing. This constitutes sale, which is a refinement on barter. Trade is only a step farther ; it is a double sale, the merchant buys, not for himself, but for others. A merchant is a machine of a complex nature. Do you want, he supplies you ; have you 'any superfluities, he relieves you

of them ; do you want some of the universal equivalent money, he gives it you, by creating in you a credit in proportion to your circumstances. The introduction of so useful a machine,

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prompts every one to wish for the power of using it; and this is the reason why mankind extend their labour beyond the mere supply of their physical wants.

Trade therefore abridges the tedious operations of sale and barter, and brings to light many things highly important for individuals, who live by relieving the wants of others, to know. It makes the standard of demand, which is, in a manner, the voice of the ftatesman, conducting the operations of industry towards the relief of wants; and directing the circulation of subsistence towards the habitations of the neceffitous.'

We have given this specimen of our author's manner as a proof how well he understands his subject ; and to thew, that the science of government is a study far from being so simple, or so easily attained, as is commonly imagined,

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II. The History of the present State of Ele&ricity, with Original Expe-
riments, by Joseph Priestley, L. L. D. F. R. S. 410. Price
1!. 15.

Cadell.
O branch of natural philosophy hath been so considerably

extended and improved by the philosophers of the present age, as that which is called electricity. These improvements and discoveries have been gradual ; they were made at different times, by different people, and in different countries, and consequently are only to be learnt from a great variety of books in various languages. It must, therefore, necessarily be very agreeable to those who are engaged in electrical enquiries, to see the whole collected into one point of view, by a candid and judicious writer, who is perfectly acquainted with the subject.

Dr. Priestley divides his work into eight parts; the first of which contains the history of the discoveries of all the celebrated electricians, in the order of time in which they were published to the world. This history he divides into ten periods. The first of these comprehends the discoveries prior to those of Mr. Hawkesbee. In the second we are presented with those of Mr. Hawkesbee himself. The third period contains the discoveries of Mr. Stephen Grey, prior to those of Mr. Du Faye, which bring the history of electricity to the year 1733. In the fourth period we find the discoveries of Mr. Du Faye. Period the fifth continues and concludes Mr. Grey's experiments. Those of Dr. Desaguliers constitute the fixth. In period the seventh we are presented with the experiments of the Germans, together with those of Dr. Watson, previous to the 4

discovery

discovery of the Leyden phial in the year 1746. Period the eighth contains the history of electricity from the Leyden phial, to the discoveries of Dr. Franklin. The ninth displays the discoveries and experiments of the last mentioned incomparable electrician; and in the tenth, the history is brought down to the year 1766. It is sufficient to observe, relative to this first part of the work, that it contains a very accurate and faithful history of the birth and progress of this youngest fifter of the sciences, related in such a manner as to be at once both entertaining and instructive.

The second part comprehends a series of propositions, in which all the general properties of electricity are comprised. In part the third we find the different theories of various authors who have written on this subject, particularly those of positive and negative electricity, and of two electric fluids. Part the fourth contains Defiderata in electricity, and hints for the further extension of it. These hints, for the promotion of further discoveries, we shall transcribe.

" I. Concerning the Electrical fluid. • What is the proportion of the several colours in electric light, in different cases, and in different appearances of it?

• Is not the electric light a real vapour ignited, similar to - that of phosphorus ; and may not experiments be, hereafter, made, where we shall have the explosion, the shock, and the other effects of ele&ricity, without the light? Is the electric light ever visible except in vacuo ? In the open air the electric fluid makes itself a vacuum in order to its passage.

· Collect the electric fuid, not from the general mass of the earth but from bodies of particular kinds, and observe if it have any different properties, with respect to light; &c.

* Is it exactly the same at sea, as on land ; below the surface of the earth as above it, &c. &c. &c. ?

• Dr. Franklin observed, that iron was corroded by being exposed to repeated electric sparks. Must not this have been effected by some acid ? What other marks are there of an acid in the electric matter? May not its phosphoreal smell be reckoned one ? Is it not posible to change blue vegetable juices into red by some application of electricity ? This, I think, I have been told has been done at Edinburgh. • Is there only one electric fiuid, or are there two i Or is

electric fluid sui generis, at all, distine from the ether of Sir Ifaac Newton? If there be, in what respect does it differ froin the ether?

· Are the particles which affect the organ of smelling, as well as the particles of light, parts of the proper electric fiuid,

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or are they merely adventitious, being, some way or other, brought into action by electricity ?

• Does not some particular order of the particles, which Sir Isaac Newton supposes to be continually flying from the surfaces of all bodies, constitute the electric Auid ; as others, he impagined, constituted the air, and others the ether, &c. ?

• Is it probable that there is even any temporary, or growing addition to, or dimunition of the whole stock of electricity?

· Whence arises the elasticity of the electric fluid, and according to what law do its particles repel one another ? Mr. Price.

• II. Concerning Electrics and Conduāors. ? In what does the difference between electrics and conductors consist? In other words, what is it that makes some bo, dies permeable to the electric fluid, and others impermeable to it?

• Are the pores of electric bodies smaller than those of conductors, and do they contain very much, or very little of the electric fluid ?

. What is it in the internal structure of bodies that makes them break with a polish ? Perhaps all folid electrics do so.

• Has elasticity any connection with electricity, fome electrics being extremely elastic ?

" What is the reason why, in some of Mr. Hawkesbee's experiments, the electric light was visible through a considerable thickness of very opaque electrics, as rosin, fulphur, pitch, &c; but not through the thinneft metallic conductors ?

• What fimilarity is there in the processes of calcination, vegetation, animalization, and in some measure chrystalization; since all bodies which have gone through any of those processes, and perhaps no others, are found to be electrics?

Are not both electrics and conductors more perfe&t in their kind in proportion to their specific gravity ?

• Will not water conduct electricity the best in its state of greatest condensation; and metals the least in their greatest expansion, as shown by a pyrometer ?

* Try the conducting power of different metals, by sending a large shock through wires of the fame fize, and observing the different lengths that may be melted of the different wires. Dr. Franklin.

Compare the invisible effluvia of water with the inviâble effluvia of a burning candle, and also those proceeding from other bodies, with respect to their power of conducting elec. tricity..

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Observe what degree of heat will discharge any given degree of electricity, in order to find in what de ree heat makes air a conductor.

* III. Concerning Excitation. • What is the difference, in the internal structure of electrics, that makes some of thein excitable by friction, and others by heating and cooling ?

• What have friction, heating, cooling, and the separation after close contact in common to them all ? How do any of them contribute to excitation ? And in what manner is one, or the other kind of electricity produced by rubbers and electrics of different surfaces ?

• Is not Æpinus's experiment of pressing two flat pieces of glafs together, when one of them contracts a positive and the other a negative electricity, similar to the experiments of Mr. Wilke, concerning the production of electricity by the liquefaction of various substances in others; when the substance which melts and contracts is in one state, and that which contains it is in the opposite ? And are not both these cases similar to the excitation of the tourmalin, &c. by heating and cooling? In this case, may not the tourmalin and the air act upon one another and be in opposite states ?

• Is not the circumstance common to all these cases, some affection of that space near the surface of the bodies in which the refractive power lies? When bodies which have been pressed together within that space recede from one another, more surface, and consequently more of that space is made, doth not the electric fluid flow into it from that body which has the least power of retaining it, and which it can permeate with the moft ease ; when not being able to enter the substance of the other it rests upon its surface ?

Are not the particles of the electric and rubber thrown into a vibration in the act of excitation, which makes frequent recedings of the parts from one another, and thereby promotes the effect above-mentioned ?

• What is the real effect of putting moisture or amalgam upon the rubber? Do not those substances increase the power of excitation, as conductors more distant from the smooth glass, in the gradation of electrics, than the surface of the leather ? Or. do they only make the rubber touch in more points, or alter the surface of the rubber?

• Has that difference of surface on which colour depends any influence upon the power of excitation ?

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