118 PROCEEDINGS OF SECTION B. 
He established these points : — 
1. “Substances burn only in pure air. 
2. “This air is consumed in the combustion, and the- 
increase in weight of the substance burnt is equi- 
valent to the decrease in weight of the air. 
3. “ The combustible body is, as a rule, converted into» 
an acid by its combination with the pure air, but 
the metals, on the other hand, imto metallic 
oxides.” (See Ramsay.) 
It is a wholesome exercise to follow the steps of this 
vigorous investigation into the nature of combustion that 
began with Boyie in the middle of the 17th century, and 
finished with Cavendish and Lavoisier and their associates 
towards the end of the 18th century. It was a scientific 
crusade, m which many acute thinkers and able experiment- 
alists took part; and, when we consider the large issues in- 
volved, it constitutes one of the most deeply interesting, in- 
deed exciting, chapters of scientific history. As the history 
of this crusade has been so recently and so ably told by 
Ramsay and others, it will suffice if I simply refer in passing 
to some of the chief steps that led us into the full light. I 
shall have to point out that, about 100 years later, towards’ 
the end of the century that has just closed, this investiga- 
tion into the composition of the atmosphere was taken up 
afresh, and pursued with an address and a success that re 
minds one of that brave struggle with the mystery of the air 
that illumined and will forever make memorable the closing 
quarter of the 18th century. 
When we make these brilliant investigators repeat agaim 
before us their classical experiments, as in a mental cmema- 
tograph, we are filled with an admiration of their expert 
mental skill, as well as of their remarkable sagacity; at the 
same time, it is often very tantalising. They are all attack- 
ing the same problem; how difficult it is we can scarcely 
realise to-day. Now it is Boyle, with the wide outlook of 
genius, who is so near the central truth ;- now it is the lam~ 
ented Mayow, who has almost discovered oxygen in his “ fire- 
air,’ when he drops by the way; again, it is the botanist 
Hales, who has his gases separated out, and only requires 
to discriminate between them by a few simple tests; then 
Priestley (1733-1804) and Scheele, and Cavendish (1731- 
1810), who did so much brilliant and solid work that will 
endure; and, had they not been handicapped by the weight 
of that incubus—the Phlogistic Theory—they would dtubt- 
less have put the finishing-touches to their work. How hard 
some theories die! There must have been some wonderful 
