Farmyard Manure. 17 



These are Mr. M'Dougall's own words respecting the theory 

 of the action of his disinfecting powder. The passage cited will 

 be found (pp. 18, 19) in Mr. M'Dougall's pamphlet, entitled, 

 1 On the Preservation of the Natural Manures, by Alexander 

 M'Dougall. 1856.' 



In page 20 of this pamphlet it is said " Theoretically, it is 

 perfect, leaving nothing to be desired; and in practice, it has 

 not fallen short of the just expectations which were formed of 

 its probable results in actual use/' 



I regret that I cannot share this opinion, for Mr. M'Dougall's 

 powder is neither theoretically perfect, nor does it answer in 

 practice the purpose for which it is recommended to the notice 

 of agriculturists, for it is destitute of the property of fixing any 

 free ammonia in liquid manure or in dung-heaps. 



It is not my intention to criticise in detail Mr. M'Dougall's 

 " perfect theory, which leaves nothing to be desired ;" but I 

 trust he will excuse me for reminding him that when two or 

 more elements unite together chemically, a new compound sub- 

 stance is produced, which possesses properties not shared by its 

 constituents. Thus sulphuric acid uniting with lime produces 

 sulphate of lime, a combination in which neither the most 

 striking characters of sulphuric acid nor of lime are any longer 

 perceptible. 



In the same manner sulphurous acid uniting chemically with 

 lime produces a new compound substance, in which the most 

 prominent features of lime and sulphurous acid have become 

 permanently altered. Unless it can be shown experimentally 

 that the action of sulphurous acid in combination with lime and 

 magnesia upon sulphuretted or phosphoretted hydrogen is the 

 same as that of free sulphurous acid, Mr. M'Dougall's attempted 

 explanation of the action of the disinfecting powder upon 

 sulphuretted and phosphoretted hydrogen must indeed be re- 

 garded as a theory a theory, however, which I imagine every 

 sound chemist will more likely call a wild than a perfect one. 

 M'Dougall's powder possesses the power, though only in a 

 slight degree, of removing sulphuretted hydrogen from liquids. 

 This property it owes not to the sulphite of lime or magnesia 

 which it contains, but, as it appears to me with much greater 

 probability, to the free lime which enters into the composition 

 of M'Dougall's powder. 



In order to decide positively this point, the following experi- 

 ment was made : 



To a strong and clear solution of M'Dougall's powder in 

 water a small quantity of sulphuretted hydrogen water was 

 added ; the smell disappeared, no deposit of sulphur was pro- 

 duced. Some more sulphuretted hydrogen water was added to 



