28 
Sir Francis Darwin. 
On this basis Hales formed his theory of vegetable nutrition, 
concluding (p. 211) “ that particles of air in a fixt state” adhere to 
and are “ wrought into the substance” of plants. 1 
It is quite clear therefore that for Hales the entry of air into 
leaves was a process of nutrition. 
To discover whether Hales distinguished between assimilation 
and respiration, we must know something of the views current in 
his day. The history of our knowledge on respiration is given in 
Michael Foster’s most interesting Lectures on the History of 
Physiology (1901). It is there shown how large a share Englishmen 
had in the unravelling of the mystery of respiration, and it can 
hardly be doubted that Hales knew the writings of Lower and 
Hooke on this subject; he certainly knew Mayow’s. Lower had 
shown that in respiration air is absorbed by the blood. And he 
concluded that it is as much needed for animal life as for the burn¬ 
ing of a candle—“ in fact where a fire burns readily, there can we 
easily breathe.” 2 
Mayow’s contribution (published in 1668 when he was 25 years 
of age) was to show that only a part of the inspired air was 
essential for breathing, and by this part, “ though he called it by a 
different name, he meant what we now call oxygen.” 3 He studied 
the action of oxygen, or as he called it the “ igneo-aerial particles ” 
of air, by experiments on combustion. His later experiments show 
that he considered the burning of a candle and the respiration of an 
animal to be comparable. But he did not for this reason exclude 
the conception of assimilation. Indeed the contrary seems more 
probable, for he says that a flame enclosed in a flask “ goes out not 
because it is suffocated by its own smoke, as some have thought, 
but because it is deprived of its aerial sustenance or food (pabulum).” 4 
In describing the case of “a small animal and a lighted candle ... 
shut up in the same vessel,” he remarks (p.192) that the animal lives 
after the candle has gone out. “ So soon ” he says “ as the igneo- 
aerial particles begin to reach the flame scantily and slowly, it is 
soon extinguished. For animals, on the other hand, a lesser store 
of the aereal food 5 is sufficient, and one supplied at intervals, so that 
1 He is here especially referring to the nutrition of apples, but on the same 
page he expresses his general belief that “ air makes a very considerable part 
of the substance of Vegetables, as well as of Animals.” 
2 Quoted by Foster, p. 184. 
3 Foster, loc. cit., p. 186. 
4 Foster, loc. cit., p. 188. 
* Italics mine. 
