The absorption of water by colloids (including gelatine) is accompanied by contraction of volume (compression) of the water absorbed, and by evolution of heat, and, as has been pointed out by Koerner,[54] it is opposed (and swelling decreased) by increase of temperature. Solution, on the other hand, absorbs heat, and is therefore favoured by rise of temperature. [54] Beiträge zur wissenschaftlichen Grundlage der Gerberei, Freiberg, 1899. If we place the swollen jelly in alcohol, it parts with water and contracts. The gelatine and alcohol are not mutually soluble, the sum of the attraction of water for alcohol, and the cohesive attraction of the gelatine is greater than the attraction of the latter for water, and as the alcohol cannot pass into the gelatine, the water passes out, and the jelly contracts. The greater the concentration of the alcohol, the more completely is the jelly dehydrated, and in strong alcohol it may become quite hard and solid. If we like to express the same facts in language more familiar to the modern chemist, but perhaps less clear to the non-chemical reader, we may say that the alcohol exerts an osmotic pressure outside the gelatine, but little or none inside it, and therefore the water is squeezed out. It would be equally true to say that the water passes out of the jelly till its osmotic pressure is equal in both the jelly and the alcohol. The jelly is a true “solid solution” of water in gelatine, and in a solution we may regard either of the two constituents as the solvent. Exact parallels may be found in the distribution of a third substance between two immiscible solvents (see p. 76), say alcohol between water and ether. The osmotic pressure of water into alcohol may be demonstrated in a very simple way, taking advantage of the fact that a film of jelly is permeable for water but not for alcohol. If the experiment described on p. 78 be made by placing alcohol in a cell previously washed out with a gelatine solution, and the cell be placed in water, the water will pass into the cell, and the alcoholic solution will rise many feet in the vertical tube. The insolubility of gelatine in alcohol may be made use of for its estimation. If three times its volume of absolute alcohol be added to a solution containing gelatine, the latter will separate as a solid mass on a stirring rod, or on the sides of the beaker, and may be washed with further portions of alcohol. The method is useful in the analysis of gelatine lozenges and “jelly squares,” roller compositions, hectograph masses, and the like; and for the determination of true unaltered gelatine in glues, and commercial gelatines (see page 60). Many other colloids are however also precipitated by alcohol.