999 



5. In the majority of cases, some thickening agent is subsequently 

 added, to impart consistence. 



And, during the whole process, a temperature of about 60° is kept 

 up. 



" To analyse these various steps of the process : the preparatory 

 cleaning is rendered necessary, by the intimate connexion which sub- 

 sists between lichens and their bases of support, many species cor- 

 roding and disintegrating even the hardest quartz ; hence, many 

 (especially pulverulent and crustaceous species) require a lengthy 

 steepiog and washing in water, to free them from adherent earthy 

 impurities. The drying is merely to facilitate the next step, or pul- 

 verization, the object of which is to expose, to mechanical and che- 

 mical agents, during maceration, the greatest possible extent of surface. 

 The steeping of the powdered plant in water, or its formation there- 

 with into a pulp, assists the subsequent action of the ammoniacal 

 macerant on its particles. Ammonia is the alkali generally employed, 

 in some shape, for causing the development of colour, because expe- 

 rimentally found most uniformly suitable therefor. It is added in 

 small quantity, and from time to time, to supply the loss constantly 

 occurring, from its great tendency to volatilize, especially in the state 

 of free exposure to the air, in which the pulpy mass is kept. The 

 mixture is constantly stirred, for the purpose of more fully exposing 

 every part of it to the action of atmospheric oxygen. The 

 thickening agents sometimes added, towards the end or after the ter- 

 mination of the process of manufacture, are usually genuine adulte- 

 rations ; but they are, also, sometimes added merely to impart con- 

 sistence, thereby facilitating the making up of the mass into balls, 

 cakes, or lumps, for more easy and convenient preservation. The 

 continued application, during the whole process, of a moderate amount 

 of heal, is a point of great importance. Westring found, as the result 

 of a long series of experiments with a view to determine the effect of 

 heat in the elimination of these colours, that he could, at pleasure, 

 increase or diminish their brilliancy, or vary their tint, according as 

 he macerated in hot or cold liquids. Though I have not been able 

 to verify all Westring's special results, still 1 quite agree with him in 

 the general ones. 1 have repeatedly had occasion to observe, how- 

 ever, that, while a continued moderate degree of heat was highly 

 conducive to the colour-development, a very slight elevation of 

 temperature caused immediate deterioration ; and, in such cases, 

 cold maceration of the same lichen was invariably more successful in 

 its results. My own experiments show that, up to a certain point, 



