HISTORICAL AND INTRODUCTORY 3 



once more and got the same 200 pounds that I started with, 

 less about two ounces. Therefore the 164 pounds of wood, 

 bark, and root, arose from the water alone." 



The experiment is simple and convincing, and satisfied 

 Boyle (50), who repeated it with " squash, a kind of Indian 

 pompion " and obtained similar results. Boyle further dis- 

 tilled the plants and concluded, quite justifiably from his prem- 

 ises, that the products obtained, " salt, spirit, earth, and even 

 oil (though that be thought of all bodies the most opposite to 

 water), may be produced out of water ". Nevertheless, the 

 conclusion is incorrect, because two factors had escaped Van 

 Helmont's notice the parts played by the air and by the 

 missing two ounces of soil. But the history of this experiment 

 is thoroughly typical of experiments in agricultural chemistry 

 generally : in no other subject is it so easy to overlook a vital 

 factor and draw from good experiments a conclusion that 

 appears to be absolutely sound, but is in reality entirely 

 wrong. 



Some years later about 1650 Glauber (107) set up the 

 hypothesis that saltpetre is the "principle" of vegetation. 

 Having obtained saltpetre from the earth cleared out from 

 cattle sheds, he argued that it must have come from the urine 

 or droppings of the animals, and must, therefore, be contained 

 in the animal's food, i.e. in plants. He also found that addi- 

 tions of saltpetre to the soil produced enormous increases in 

 crop. He connected these two observations and supposed 

 that saltpetre is the essential principle of vegetation. The 

 fertility of the soil and the value of manures (he mentions dung, 

 feathers, hair, horn, bones, cloth cuttings) are entirely due to 

 saltpetre. 



This view was supported by Mayow's experiments (195). 

 He estimated the amounts of nitre in the soil at different times 

 of the year, and showed that it occurs in greatest quantity in 

 spring when plants are just beginning to grow, but is not to 

 be found " in soil on which plants grow abundantly, the reason 

 being that all the nitre of the soil is sucked out by the plants ". 

 Kiilbel (quoted in 293), on the other hand, regarded a magma 



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