HISTORICAL AND INTRODUCTORY 5 



mentioned are, however, too important to be passed over completely. 

 Wallerius, in 1761, professor of chemistry at Upsala, after analysing 

 plants to discover the materials on which they live, and arguing that 

 Nutritio non fieri potest a rebus heterogeneis, sed homogeneis^ concludes 

 that humus, being homogeneis, is the source of their food the nutritiva 

 while the other soil constituents are instrumentalia^ making the proper 

 food mixture, dissolving and attenuating it, till it can enter the plant 

 root. Thus chalk and probably salts help in dissolving the " fatness " 

 of the humus. Clay helps to retain the " fatness " and prevent it being 

 washed away by rain : sand keeps the soil open and pervious to air. 

 The Earl of Dundonald, in 1795, adds alkaline phosphates to the list 

 of nutritive salts, but he attaches chief importance to humus as plant 

 food. The " oxygenation " process going on in the soil makes the 

 organic matter insoluble and therefore useless for the plant ; lime, 

 " alkalis and other saline substances " dissolve it and change it to plant 

 food ; hence these substances should be used alternately with dung as 

 manure. Manures were thus divided, as by Wallerius, into two classes : 

 those that afford plant food, and those that have some indirect effect. 



Throughout this period it was believed that plants could generate 

 alkalies. "Alkalies," wrote Kirwan in 1796, "seem to be the pro- 

 duct of the vegetable process, for either none, or scarce any, is found 

 in the soils, or in rain water." In like manner Lampadius thought 

 he had proved that plants could generate silica. The theory that 

 plants agreed in all essentials with animals was still accepted by many 

 men of science ; some interesting developments were made by Erasmus 

 Darwin in 1803 (76). 



Between 1 770 and 1 800 work was done on the effects of vegetation 

 on air that was destined to revolutionise the ideas of the function of 

 plants in the economy of Nature, but its agricultural significance was 

 not recognised until later. In 1771 Priestley (229), knowing that the 

 atmosphere becomes vitiated by animal respiration, combustion, putre- 

 faction, etc., and realising that some natural purification must go on, 

 or life would not longer be possible, was led to try the effect of sprigs 

 of living mint on vitiated air. He found that the mint made the air 

 purer, and concludes " that plants instead of affecting the air in the 

 same manner with animal respiration, reverse the effects of breathing, 

 and tend to keep the atmosphere pure and wholesome, when it is 

 become noxious in consequence of animals either living, or breathing, 

 or dying, and putrefying in it ". But he had not yet discovered 

 oxygen, and so could not give precision to his discovery : and when, 

 later on, he did discover oxygen and learn how to estimate it, he- 



