298 DYEING 



constituents by dissolving in them will here be called a lysochrome 

 (Greek, hisis, solution). Since these colorants find scarcely any 

 application except in the study of lipids, the word will be used by 

 itself in what follows to mean lipid-soluble lysochromes, unless the 

 contrary is indicated. 



To present a lysochrome to a section or other microscopical 

 preparation, it is necessary to use a suitable solvent. Lysochromes 

 are freely soluble in benzene, chloroform, ether, etc., but these 

 solvents are unusable, since they would dissolve out the tissue- 

 constituents that it is desired to show. A solvent that will not dis- 

 solve lipids is required. It is important also that the colouring agent 

 should be less soluble in the solvent than in lipids. An example of a 

 fluid fulfilling these requirements is 70% ethanol. If the lysochrome 

 is presented to the tissues as a solution in this solvent, a partition 

 will take place, some of it remaining in the solvent and the rest dis- 

 solving in the lipid. The amount of colour shown by the lipid will 

 be governed by the partition-coefiicient applying to the particular 

 colouring agent in the presence of the two solvents concerned. 

 Ideally the coefficient should strongly favour the lipid. 



If a preparation that has been coloured in this way is placed in 

 70% ethanol (or other such solvent), the minute amount of lyso- 

 chrome contained in the lipid tissue-constituents has to undergo 

 partition between this lipid on one hand and the whole of the sol- 

 vent in the jar on the other. This partition greatly favours the 

 solvent, on account of its immense superiority in volume. As a 

 result, no detectable amount of lysochrome remains in the lipid. 

 There are no electrostatic or other bonds to hold the lysochrome 

 in position. No more than a dip in the solvent is permissible (to 

 prevent precipitation of the lysochrome, which is usually dissolved 

 in the ethanol near saturation); the preparation must then be 

 brought quickly into a fluid in which the colour is insoluble. 



It follows from what has been said that a Ivsochrome, in the 

 widest sense, must fulfil the following requirements. It must 



(i) be strongly coloured; 



(2) be very soluble in the substances it is intended to show; 



(3) have no capacity to attach itself to any tissue-constituent 

 except by solution; 



(4) be capable of presentation to the tissues in a solvent 

 having the following characters, {a) The solvent will not 

 dissolve the substances the lysochrome is intended to 



