282 TECHNICAL MICROSCOPY. 



Instead of the usual thick balsam, a weak solution in ether 

 or chloroform is, under certain circumstances, advantageously 

 employed. Used cold, it may be spread on the slide with a brush; 

 the preparation is laid on, more liquid is applied, and it is then 

 finally covered. 



(3.) Glycerine, for most vegetable tissues. This is specially recom- 

 mended for preparations where the preservation of starch-grains 

 and chlorophyll is of importance. In order to limit the contrac- 

 tion of the primordial utricle as much as possible, the objects are 

 first placed in a very weak solution with distilled water, which is 

 either allowed to evaporate in the air or is gradually replaced by 

 more concentrated solutions. The property of glycerine of absorb- 

 ing water from the air up to a certain degree of concentration 

 renders hermetic sealing superfluous ; where, however, it is to 

 remain constantly mixed with a rather large quantity of water, 

 evaporation must be prevented by careful application of the 

 varnish mentioned later on. There is no need for haste in this 

 process ; several preparations may be collected before we proceed 

 to cement them. 



Instead of pure glycerine others employ a mixture of glycerine 

 and gum arabic or gelatine, because subsequent change of place of 

 the objects, through the solidifying of this liquid, is prevented. 

 This mixture is obtained by heating one part of gelatine with two 

 parts of water, and then adding an equal volume, or even slightly 

 more, of glycerine. 



Farrants recommends a still more complex mixture, consisting 

 of equal parts of glycerine, gum arabic, and a saturated solution of 

 arsenious acid ; it is'used like Canada balsam. Cementing is not 

 here necessary, as the outer layers soon become hard and prevent 

 further evaporation. 



(4.) Chloride of calcium, for most vegetable tissues, is employed in 

 a similar manner to glycerine ; if sufficiently concentrated, like 

 glycerine, it does not need hermetic sealing. By carefully increas- 

 ing the strength of the solution, the walls of the cells, even in 

 young tissues, may generally be preserved ; the colouring-matters, 

 however, become more or less altered, the starch-grains swell, and 

 the primordial utricle contracts. Nevertheless, chloride of calcium 

 solution is in many cases to be preferred to glycerine. To prevent 

 the formation of basic chloride of calcium, which has a tendency to 

 crystallize on the preparation, the solution may be acidified with 



