^54 THE HISTORY OF BIOLOGY 



sorbed by the plant through the leaves and are converted into solid sub- 

 stances. Likewise he states, though with less certainty, that light penetrates 

 the leaves and co-operates in the alimental processes in them. In these asser- 

 tions attempts have been made to find definite proof of Hales's genius, and 

 there is undeniably in them a brilliant guess at facts that were established 

 at a later period, but in regard to points of detail his speculations on the 

 properties of air are undoubtedly far more deficient than his quantitative 

 experiments. It is true that gas-chemistry had been but little developed in 

 his time, but it would seem that he scarcely took advantage of what actually 

 was known; he certainly cites Boyle quite frequently, but he evidently knew 

 nothing of van Helmont's gas-experiments. To him all gases are "air," both 

 that which arises from the dry distillation of wood and that which is formed 

 by treating lime with acid. In such circumstances it was inevitable that the 

 great trouble he went to in investigating the influence of the air upon vege- 

 table, and even animal, life was to a great extent in vain. What he achieved 

 as an experimenter, however, is quite enough to ensure for him considerable 

 fame in the history of biology, and it was to be long before science advanced 

 beyond his point of view. For this to happen there was required above all 

 else a reformation of the science of chemistry — which indeed actually took 

 place at the end of the eighteenth century. This will be described in a fol- 

 lowing chapter. During the latter part of his life Hales also worked at ex- 

 periments on animals, particularly in regard to the blood-circulation, and he 

 displayed in this sphere the same power of arranging experiments and draw- 

 ing conclusions therefrom that he showed in his botanical investigations. 

 He measured the blood-pressure in live mammals by introducing into a vein 

 a tube in which the blood was made to rise; he calculated the speed of the 

 blood-stream in the veins and capillaries from the volume of the vessels, 

 the rate of movement of the blood-mass, and the resistance of the walls. 

 To these investigations he added a quantity of notes on medicinal and hy- 

 gienic subjects, with particular reference to the injuriousness of alcoholic 

 liquors, for he was a keen supporter of temperance. This fact gives his 

 Hamasfaficks, as he called his investigations into the blood, a far more 

 motley character than his treatise on vegetable physiology; nevertheless, 

 even these investigations are of some value and he holds a place of honour in 

 the history of physiology. 



Among the plant-physiologists who, after Hales, distinguished them- 

 selves during the eighteenth century there are one or two who carried out 

 important experimental observations regarding plant-reproduction, and who 

 deserve special mention. 



Joseph Gottlieb Koelreuter was born at Sulz, Wiirttemberg, in 1733. 

 We know, on the whole, very little about his life; he seems to have studied 

 in Berlin and Leipzig and spent some time in St. Petersburg; in 1764 he was 



