PHYSICAL SCIENCE. 



li 



Sir Humphry Davy made a most important I 

 improvement in this theory of attraction and de- 

 composition, and first rendered the way in which 

 decomposition takes place intelligible. The 

 atoms of bodies, according to him, are always in 

 an electrical state ; some of them being positive 

 and others negative. Bodies in the same state 

 hate no affinity for each other, and, of course, do 

 not unite, unless one of them be much more 

 powerfully positive or negative than the other. 

 Whon this happens, and the two atoms are 

 brought near each other, the opposite state may 

 be produced in the other by induction, in which 

 case union may take place. Thus oxygen and 

 chlorine are both negative ; but oxygen much 

 more so than chlorine. Hence when they ap- 

 proach near each other, the negative state of the 

 former will increase, while that of the latter 

 will diminish, so that it may become actually posi- 

 tive, though only weakly so. Hence they will 

 unite ; but the union will not be strong. 



Bodies in different states have an affinity for 

 each other, and this affinity augments in propor- 

 tion to the intensity of the two states of electri- 

 city in the two bodies. Thus oxygen is in- 

 tensely negative, and hydrogen is intensely posi- 

 tive. Hence there is a strong affinity between 

 them. Sulphuric acid is negative, and magnesia 

 positive ; hence they unite, and constitute sul- 

 phate of magnesia. Potash is more strongly 

 positive than magnesia. Hence when ifeis mixed 

 with sulphate of magnesia, it will not only attract 

 the sulphuric acid more strongly than the mag- 

 nesia does ; but it will, at the same time, repel 

 the magnesia, because both are in the same 

 electrical state. This greater attraction, com- 

 bined with the repulsion, is capable of driving 

 off the magnesia, and enabling the potash to 

 lake its place. 



The first attempt to generalize the phenomena 

 of chemistry was made by Beccher, and by his 

 successor Stahl. Combustion was the most ex- 

 traordinary phenomenon in chemistry. It was 

 confined to a certain set of bodies called com- 

 bustibles. When these bodies were heated in 

 the open air, they became hotter of themselves, 

 and continued for a certain time to give out 

 heat and light in great abundance. When the 

 process is at an end, these bodies have lost their 

 original appearance, and have assumed another 

 quite different. Thus when sulphur is burnt, 

 it is converted into an invisible gaseous body, 

 having a strong and suffocating smell, and a sour 

 taste, and known by the name of sulphurous acid. 

 Phosphorus is changed into a white solid sour 

 tasted body called phosphoric acid ; iron into a 

 red powder, lead into a red or yellow powder 

 and mercury into a black or red powder. Beccher 

 and Stahl thought that all combustible bodies 



owe their combustibility to the presence of a 

 peculiar body, which was the same in all, and to 

 which they gave the name of phlogiston, (or the 

 combustible body.) During the burning of the 

 combustible body the phlogiston makes its escape, 

 and what remains is the other constituent : foi 

 all combustibles, in their opinion, consisted or 

 two constituents, phlogiston and an incombustible 

 substance which varied for each body. For sul- 

 phur, it was sulphurous acid ; for phosphorus, 

 phosphoric acid. For iron, lead, and mercury 

 the black, yellow, or red powders which remain 

 when these metals are calcined. These opinions 

 were universally adopted by chemists, and modi- 

 fied their views, and regulated their mode of 

 experimenting. Boyle, indeed, had shown that 

 tin, when calcined, increases in weight ; but this 

 did not alter their opinions, because the increase 

 of weight was ascribed to fire fixed in the tin 

 during the process of calcination. 



Chemistry continued in this country little else 

 than the art of preparing medicines, till Dr 

 Cullen began his great career about the year 

 1740. He was aware of the important purposes 

 to which the science might be applied, and had 

 devoted himself to the subject In 1746 he was 

 appointed lecturer on chemistry in the college 

 of Glasgow ; and though his attention was 

 speedily withdrawn from his favourite science, 

 by his being placed in the medical chair of that 

 university, yet he had the merit of rendering 

 chemistry popular, and of educating Dr Black, 

 who became his favourite pupil and friend. 



Black, when a very young man, discovered 

 that limestone is a salt composed of two different 

 constituents, namely, lime, which possesses the 

 characters of an alkali or base ; and carbonic 

 acid, which possesses the characters of an acid. 

 This last ingredient is a gas or aerial body when 

 in a separate state. Hence it is disengaged 

 when lime is dissolved in an acid, and occasions 

 the effervescence which always accompanies such 

 solutions. When the limestone is exposed to a 

 strong heat, the carbonic acid is, in like manner^ 

 driven off, and leaves the lime in a separate 

 state. What is called quicklime, or burnt lime 

 is the pure substance, or caustic lime. When 

 combined with carbonic acid it loses its causti- 

 city, and is converted into a mild tasteless salt. 



Mr Cavendish took up the subject of carbonic 

 acid where Dr Black had laid it down. He 

 determined its specific gravity, its solubility in 

 water, and its various other properties. Berg- 

 mann dwelt upon its acid characters, and describ- 

 ed the salts which it forms, with the various bases. 

 Dr Priestley made experiments upon its anti- 

 septic properties, and, together with Dr M'Bryde, 

 recommended water impregnated with it as a 

 valuable medicine. Lavoisier determined iis 



