lii 



THE PROGRESS OF 



nature, showed that it was funned during the 

 combustion of charcoal, and that it was a com. 

 pound of three parts by weight of charcoal, and 

 eight parts of oxygen. Hence the reason of its 

 formation during the combustion of wood and 

 coal, and all those bodies which contain charcoal. 



Mr Cavendish examined the properties of 

 hydrogen gas, showed how to prepare it by 

 dissolving iron or zinc in dilute sulphuric acid, 

 determined its sensible qualities, ascertained its 

 specific gravity, examined its combustibility, and 

 discovered that, when burnt, it unites with half 

 its volume of oxygen gas, and that the product 

 is water. Tin- subsequent investigations of Dr 

 1'rout and Dr Thomson have shown that oxygen 

 gas is sixteen times heavier than the same bulk 

 of hydrogen gas. Hence, as water is a com- 

 pound of one volume of oxygen, and two volumes 

 of hydrogen, it follows that water contains $tbs 

 of its weight of oxygen, and ^th of hydrogen. 



Dr Priestley discovered oxygen gas on the 

 4th of August, 1774, by heating the red oxide of 

 mercury in a gun barrel. He found it to be a 

 gas; that bodies burn in it better, and that ani- 

 mals are capable of breathing it longer than 

 common air. He considered common air as a 

 compound of oxygen and phlogiston. When 

 air is saturated with phlogiston, he thought it 

 MOS converted into azotic gas, which therefore he 

 oiled phlogisticaled air. For a similar reason 

 he called oxygen depldogisticaled air. But 

 Scheele, who had been engaged in the same in- 

 vestigations, without being aware of the labours 

 of Priestley, took a simpler view of the subject. 

 He considered common air as a mixture of 

 oxygen and azotic gases ; both of which he 

 thought were simple bodies. It was the oxygen 

 that made air capable of supporting combustion, 

 and capable of being breathed. Lavoisier adopt- 

 ed the opinion of Scheele, and betrayed the 

 origin of his views by assuming the very same 

 proportions of oxygen and azotic gas in air that 

 Scheele had done. Cavendish, in 1783, de- 

 monstrated that air is a compound of about four 

 volumes azotic and one volume oxygen gas; 

 and he showed, by making trials in the neigh- 

 bourhood of London for a whole year, that these 

 proportions do not sensibly vary. These views, 

 after having been neglected or overlooked by 

 chemists for nearly twenty years, were univer- 

 sally adopted about the beginning of the present 

 century, chiefly in consequence of the observa- 

 tions of Berthollet. 



Chlorine was discovered by Scheele in 1774, 

 during his researches on manganese. He found 

 that it was disengaged when muriatic acid was 

 digested on the black oxide of manganese. He 

 observed its smell, its colour, and its property of 

 destroying vegetable colours ; and he considered 



it as muriatic acid, deprived of phlogiston, by 

 which term he understood hydrogen gas. The 

 properties of this gas were farther examined by 

 Berthollet and Chenevix. Berthollet considered 

 it as a compound of muriatic acid and oxygen ; 

 and, therefore, gave it the name of oxy-muriatic 

 acid. He had the merit of first proposing it to 

 bleach linen and cotton, a process which was 

 afterwards brought to perfection in Glasgow. 

 In 1609, Gay-Lussac and Thenard made a set of 

 experiments, in order to separate oxygen from 

 it ; but without being able to obtain anyevidence 

 that it contained any. The subject was taken 

 up by Davy in 1810. He was equally unable to 

 obtain any proof of the presence of oxygen in it. 

 He, therefore, affirmed that it was not a com- 

 pound of muriatic acid and oxygen ; but a simple 

 substance, similar in many of its properties to 

 oxygen itself, and, like it, capable of uniting 

 with all the other simple bodies. This induced 

 him to give it the name of chlorine, from a 

 Greek word signifying grass green. ' He showed 

 that when equal volumes of chlorine and hydro- 

 gen gases are mixed together, and an electrical 

 spark passed through the mixture, combu.stiun 

 takes place ; the two gases unite, and ore con- 

 verted into muriatic acid gas, equal in volume to 

 the bulk of the two gases before combustion. 

 Hence it follows that muriatic acid is a compound 

 of chlorine and hydrogen. Thus the theory of 

 Scheele was established by Davy. 



Bromine was discovered byM. Ba lard in 1825, 

 and iodine by M. Courtois in 1811. The former 

 is a hyacinth-red liquid, the latter a bluish vola- 

 tile solid, having the metallic lustre and some 

 resemblance to antimony in its appearance. The 

 properties of bromine were investigated by 

 Bulard, and those of iodine by Gay-Lussac and 

 Davy ; but chiefly by the former. Both of these 

 bodies are simple substances ; both resemble 

 chlorine very closely in their characters, and, 

 like it, they combine with other bodies. The 

 compounds of chlorine, bromine, and iodine, 

 with other substances, are distinguished by the 

 names of chlorides, bromides, and iodides; just 

 as the compounds of oxygen are called oxides. 



Azotic gas was first noticed by Dr Rutherford, 

 in his thesis written in 1772. Scheele published 

 his analysis of air, and his investigation of the 

 properties of azote, in the year 1777 ; and 

 Priestley investigated various of its properties. 

 But it is to Mr Cavendish that the science is 

 chiefly indebted for every thing accurate that has 

 been determined respecting this gas. He ascer. 

 tained the volume of it that exists in common 

 air, showed that it is capable of combining with 

 oxygen, and that the compound formed is nitric 

 acid. It was afterwards ascertained that the 

 specific gravity of azotic gas is 0-97:22, and that 



