Most bacteria are very susceptible to changes in the degree of acidity or alkalinity of the medium in which they grow. Some kinds prefer a slightly acid reaction, some a slightly alkaline, and some a neutral (with reference to litmus as indicator). The organism which is the commonest cause of the souring of milk thrives so well in the acid medium it produces that it crowds out practically all other kinds, though its own growth is eventually stopped by too much acid. Acid soils are usually low in numbers of bacteria and as a consequence produce poor crops. The disease-producing bacteria as a class grow best in a medium which is slightly alkaline. Accurate determination of limits have been made on but few organisms. The reaction is a most important factor in growing bacteria on artificial media (see Making of Media, Chapter XVI). INJURIOUS CHEMICAL SUBSTANCES. (SEE DISINFECTION AND DISINFECTANTS, Chapter XIII.) CHEMICAL COMPOSITION. The chemical composition is subject to wide variation chiefly for two reasons: First, the cell wall in most instances seems to exert only a slight selective action in the absorption of mineral salts so that their concentration within the cell is very nearly that of the surrounding medium. Second, the chief organic constitutents vary remarkably with the kind and amount of food material available--a rich protein pabulum increases the protein, a plentiful supply of carbohydrates or of fat results in the storing of more fat, especially and _vice versa_. These facts must be borne in mind in considering the chemistry of bacteria. Of the chemical elements known, only the following seem to be essential in the structure of bacteria: carbon, hydrogen, oxygen, nitrogen, sulphur, phosphorus, chlorine, potassium, calcium, magnesium, iron, manganese. Other elements, as sodium, iodine, silicon, aluminum, lithium, copper, etc., have been reported by different analysts, but none of them can be regarded as essential, except possibly in isolated instances. These elements exist in the bacterial cell in a great variety of combinations of which the most abundant is _water_. The amount of water varies in different species from 75 to 90 per cent. of the total weight in growing cells, and is less in spores. The amount of _ash_ has been shown by different observers to vary from less than 2 per cent. to as much as 30 per cent. of the _dry weight_. The following table compiled from various sources will give an idea of the relative abundance of the different elements in the ash. S as SO₃ 7.64 per cent. (much more in sulphur bacteria) P as P₂O₅ 18.14 „ to 73.94 per cent. Cl 2.29 „ K as K₂O 11.1 „ to 25.59 „ Ca as CaO 12.64 „ to 14.0 „ Mg as MgO 0.7 „ to 11.55 „ Fe as Fe₂O₃ 1.0 „ to 8.15 „ (iron bacteria) Mn traces