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 

 diseno-a^rement of acetic acid. Alcohol scarcely acts upon humus, 

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

 which probahly 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, 

 possessincr the characters which we have recognised in ulinine. ihe 

 ulmine which is separated in this way, is tar 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 ot 

 ashes by incineration, onlv 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 crave a new quantity of soluble matter under renewed w;ashing 

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

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

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

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

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

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



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 plant or the sap must be dried and inciner- 

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

 recrard 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 ot 

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

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

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

 nothincrmore 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 destructibihty of vegetable 

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

 as often as aoriculturists 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. 1 may add, that the nature of tlie climate has a 

 vast influence upon the dissipation of the fertilizing principles ot the 

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

 the superficial ploughings or hoeings which the land so commonly 



