CHEMISTRY. 



99 



Eiejnentj being used as detergents. They are formed whenever a 

 solution of common soap in water is mixed with that of 

 an earthy salt. Hence the reason that all waters holding 

 an earthy salt in solution are unfit for washing. They de- 

 compose the common soap, and form a soap insoluble in 

 water. Such waters are called hard, and are very fre- 

 quent, especially in pit wells. 



All the earthy soaps are insoluble in alcohol, except 

 soap of magnesia, which dissolves both in alcohol and fix- 

 ed oils. The earthy soaps are all white, and require a 

 considerable heat to melt them. 



SECT. III. Of Metallic Soapt. 



Metallic The metallic soaps may be formed in the same way as 



>? the earthy soaps. They are insoluble in water, and can- 



not be used as detergents ; but several of them are solu- 

 ble in alcohol and in fixed oils. The greater number of 

 them have a white colour ; but soap of cobalt is of a 

 leaden colour; soap of iron, reddish-brown; and soap of 

 copper, green. Berthollet, who examined these soaps, has 

 recommended some of them as paints. 



Some of the metallic oxides, as those of mercury, lead, 

 and bismuth, when mixed with fat oils and water, and 

 boiled, form an intimate combination with the oil, used 

 by surgeons under the name of plaster. Litharge is the 

 metallic substance commonly used for these compounds, 

 and olive oil answers better than any other hitherto tried. 

 These plasters soften when heated, and adhere very strong- 

 ly to the skin when spread thin upon linen or leather, but 

 they may be drawn off by using the requisite force, with- 

 out leaving any portion adhering to the skin. In thee 

 properties their excellence consists. 



BOOK III. 



OF AFFINITY. 



f Afliiutr. HATING taken a view of the different substances which 

 constitute the objects of chemistry, it remains for us to 

 make a few remarks on the force by which different bo- 

 dies are united together, and kept in combination. This 

 force has received the name of affinity. 



All the great bodies which constitute the solar'system 

 are urged towards each other by a force which preserves 

 them in their orbits, and regulates their motions. This 

 force has received the name of attraction. Newton de- 

 monstrated that this force ii the same with gravitation, 

 or the force by which a heavy body is urged towards the 

 earth. 



When two bodies are brought within a certain dis- 

 tance, they adhere together, and require a considerable 

 force to separate them. Hence it appears that bodies 

 are not only attracted towards the planetary bodies, but 

 towards each other. In all cases we find the particles of 

 matter united together in masses, differing indeed in 

 magnitude, but containing all of them a considerable num- 

 ber of particles. These particles remain united, and 

 cannot be separated without the application of a consi- 

 derable force. 



Thus we see that there is a certain unknown force 

 which urges bodies towards each other ; a force which 

 acts not only upon large masses of matter, but upon the 

 particles of which bodies are composed. Attraction, 



therefore, as far as we know, extends to all matter, and Elements 



exists mutually between all matter. 



7 ... . . .. Chemistry. 



The change which attraction produces on bodies is a -_- __'' 



diminution of their distance. Now the distances of bo- 

 dies from each other are of two kinds, either too small 

 to be perceived by our senses, or great enough to be 

 easily perceived and estimated. Hence the attractions of 

 bodies, as far as regards us, naturally divide themselves 

 into two classes. 1. Those which act at sensible dis- 

 tances from each other. 2. Those which act at insen- 

 sible distances. It is to the second of these attractions 

 that the term chemical affinity has been given. 



Chemical affinity then is the attraction which exists 

 between the particles of bodies, which urges them to- 

 wards each other, and keeps them united. Now the 

 particles of matter are of two kinds, either homogeneous 

 or heterogeneous. By homogeneous particles are meant 

 the particles which compose the same body ; by hetero- 

 geneous, those which compose different bodies. Thus 

 the particles of iron are homogeneous ; but a particle of 

 iron and a particle of lead are heterogeneous. 



Homogeneous affinity urges the homogeneous par- 

 ticles towards each other, and keeps them united. It 

 is usually denoted by the term cohesion, and sometimes 

 by adhesion, when the surfaces of bodies only are refer- 

 red to. 



Heterogeneous affinity urges heterogeneous particles 

 towards each other, and keeps them at insensible dis- 

 tances, and of course is the cause of the formation of 

 new integrant particles composed of a certain number of 

 heterogeneous particles. 



Affinity, like sensible attraction, varies with the dis- 

 tance of the attracting bodies ; but the rate at which it 

 varies remains still unknown. The characteristic marks 

 of affinity may be reduced to the three following. 



1. It acts only at insensible distances, and of course 

 affects only the particles of bodies. 



2. Its force is always the same in the same particles, 

 but it is different in different particles. 



3. This difference is not affected by the mass. Thus, 

 if A have a greater affinity for C than B has, the mass 

 of B may be considerably increase d while that of A re- 

 mains unchanged, yet B is not capable of taking a part 

 of C from A. * Let us now take a particular view of 

 gases, liquids, and solids, that we may ascertain in what 

 way they unite, and how far their combinations are in- 

 fluenced by the state of the bodies themselves. 



CHAP. I. 

 Of Gases. 



GASES are elastic fluids, which yield to the smallest 

 impression, and have their parts easily moved. Their 

 elasticity varies with the pressure, and hence it follows 

 that their particles mutually repel inversely as the dis- 

 tances of their centres from each other. The gaseous 

 bodies at present known (including some vapours) amount 

 to 22. The following Table exhibits their names and 

 their specific gravity. 



Gases. Specific Gravity. 



Air 1.000 



Chloric gas 2.713 



Nitric acid 2.427 



Sulphurous acid 2.097 



r 3 Bth!Z y 'a"? 

 illeu But t 



reived by chemists as an axiom, and illustrated by Bergman at great length. It was lately <!. 

 ; experiments on which hie opinion wa founded have not stood the test of repetition. 



