HUMUS. 247 



Weak acids have no other effect upon humus than to dissolve out 

 the metallic, earthy, and alkaline elements which it contains. The 

 more powerful acids, such as the sulphuric acid, frequently cause a 

 disengagement of acetic acid. Alcohol scarcely acts upon humus, 

 merely dissolving out of it a few hundredth parts of resinous matter, 

 which probably pre-existed in the vegetable. Potash and soda dis- 

 solve humus almost completely, causing an evolution of ammonia. 

 From this solution, acids throw down a brown, inflammable powder, 

 possessing the characters which we have recognised in ulmine. The 

 ulmine which is separated in this way, is far from corresponding 

 with the weight of the matter treated with the alkalies, which is 

 evidently due to the humus containing principles which are not pre- 

 cipitated from the alkaline solution. 



A quantity of humus which yielded no more than one tenth of 

 ashes by incineration, only lost one eleventh of its weight under re- 

 peated treatments with boiling water. The humus thus exhausted, 

 was exposed in a moist state to the action of the air for three months, 

 and gave a new quantity of soluble matter under renewed washing 

 with water; and the same effect is constantly reproduced. By ex- 

 posing moist insoluble humus to the air, therefore, a quantity of so- 

 luble extractive matter is formed. This matter, obtained by evapo- 

 rating the water which is charged with it, is not deliquescent ; it 

 yields ammonia on distillation. The watery solution, brought to the 

 consistence of sirup, is neutral to re-agents, and its taste is sensibly 

 sweet. 



It is familiarly known that the alkaline salts, which enter into the 

 constitution of vegetable juices, but rarely exhibit the reactions that 

 are proper to them ; the plan or the sap must be dried and inciner- 

 ated before their presence can be ascertained. It is the same with 

 regard to the salts contained in humus. 



Humus, as I have already observed, is the last term in the putre- 

 faction of vegetable organic matter ; its elements have acquired a 

 stability which enables them to resist all fermentation. M. de Saus- 

 sure preserved humus for a whole year in vessels filled with distilled 

 water, and plunged in mercury, without remarking any emission of 

 gas. Still it is unquestionable that the organic portion of humus is. 

 completely destructible when exposed moist to the action of the air; 

 in the course of time it is dissipated, and by and by there remains 

 nothing more than the fixed saline and earthy matters which it con- 

 tained. This fact M. B. de Saussure had already perceived from 

 his observations upon the vegetable soil that occurs in the country 

 between San Germano and Turin. This destructibiliiy of vegetable 

 earth, says M. de Saussure, sen., is a fact without exception ; and 

 as often as agriculturists have proposed to supply the place of ma- 

 nure by repeated ploughings, they have had sad experience of its 

 truth : the soil is gradually impoverished, and fertile fields ultimate- 

 ly become barren. I may add, that the nature of the climate has a 

 vast influence upon the dissipation of the fertilizing principles of the 

 soil, and that Europeans are certainly in error when they object to 

 the superficial ploughings or hoeings which the land ^ commonly 



