180 PRINCIPLES OF CHEMISTRY 



are especially subject to this action. They rot and putrefy that is, 

 their solid matter is transformed into gases, under the influence of 

 moisture, and atmospheric oxygen, and often under the influence of 

 other organisms, such as moulds, worms, micro-organisms (bacteria), and 

 suchlike. These are processes of slow combustion, of slow combination 

 with oxygen. Everyone knows that manure rots and evolves heat, 

 that stacks of damp hay, damp flour, straw, &c., become heated and 

 are changed in the process. 47 In all these transformations there are 

 formed the same chief products of combustion as are contained in 

 smoke ; the carbon gives carbonic anhydride, and the hydrogen 

 water. Hence these processes require oxygen just like combustion. 

 This is the reason why the entire prevention of access of air hinders 

 these transformations, 48 and an increased supply of air accelerates them. 

 The mechanical treatment of arable lands by the plough, harrow, and 

 other similar means has not only the object of facilitating the spread 

 of roots in the ground, and of making the soil more permeable to water, 

 but it also serves to facilitate the access of the air to the component 

 parts of the soil ; as a consequence of which the organic remains of 

 soil rot so to speak, breathe air and evolve carbonic anhydride. 

 One acre of good garden land in summer evolves more than six tons 

 of carbonic anhydride. 



It is not only vegetable and animal substances which are subject ta 

 slow oxidation in the presence of water. The very metals are rusted 

 under these conditions. Copper very easily absorbs oxygen in the 

 presence of acids. Many metallic sulphides (for example, pyrites) are 

 very easily oxidised with access of air and moisture. Thus processes 

 of slow oxidation proceed throughout nature. 



There are many elements which do not, under any circumstances,, 

 combine directly with gaseous oxygen ; nevertheless their compounds 

 with oxygen may be obtained. Platinum, gold, iridium, chlorine, 

 and iodine are examples of such elements. In this case recourse is 

 had to a so-called indirect method i.e., the given substance is- 



47 Cotton waste (it is used in factories for cleaning machines from lubricating oil) 

 soaked in oil and lying in heaps is self-combustible, being oxidised by the air. 



48 "When it is desired to preserve a supply of vegetable and animal food, the access of 

 the oxygen of the atmosphere (and also of the germs of organisms borne in the air) 

 is often prevented. For this reason articles of food ai - e often kept in hermetically closed 

 vessels, from which the air is withdrawn ; vegetables are dried and soldered up while hot 

 in tin boxes ; sardines are immersed in oil, &c. The removal of water from substances is 

 also sometimes resorted to with the same object (the drying of hay, corn, fruits), as also 

 is saturation with substances which absorb oxygen (such as sulphurous anhydride), 

 which hinder the growth of organisms forming the first cause of putrefaction, as in 

 processes of smoking, embalming, and in the keeping of fishes and other animal sj 

 mens in spirit, &c. 



