OF MICROSCOPIC OBJECTS. 21 



exposed to heat until, on cooling, it assumes a glassy appearance ; 

 it is then dissolved in pure chloroform until it becomes of the 

 consistence of thick varnish. This liquid is very convenient in 

 some cases ; as air-bubbles are much more easily got rid of than 

 when undiluted Canada balsam is used. _ t lt also dries readily, as 

 the chloroform evaporates very quickly, for which reason it must 

 be preserved in a closely-stoppered bottle. It has been said that 

 this mixture becomes cloudy with long keeping, but I have not 

 found it so in any cases where I have used it. Should it, how- 

 ever, become so, a little heat will readily dispel the opacity. 

 The ordinary balsam, if exposed much to the air whilst being 

 used, becomes thicker, as has been already stated. It may be 

 reduced to the required consistency with common turpentine, 

 but I have often found this in some degree injurious to the trans- 

 parency of the balsam, and the amalgamation of the two is by no 

 means perfect. (See also Chapter III.) The cheapness of the 

 article renders it no extravagance to use it always undiluted; 

 and when preserved in a bottle with a hollow cover fitting tightly 

 around the neck, both surfaces being finely ground, it remains fit 

 for use much longer than in the ordinary jar. 



ASPHALTUM. This substance is dissolved in linseed oil, turpen- 

 tine, or naphtha, and is often termed " Brunswick black." It is 

 easily worked, but is not generally deemed a trustworthy cement, 

 as after a time it is readily loosened from its ground. It is, how- 

 ever, very useful for some purposes (such as " finishing" the 

 slides), as it dries quickly. I shall, however, mention a modifica- 

 tion of this cement a little further on. 



MARINE GLUE. No cement is more useful or trustworthy for 

 certain purposes than this. It is made in various proportions ; 

 but one really good mixture is equal parts of india-rubber and 

 gum shellac ; these are dissolved in mineral naphtha with heat. 

 It is, however, much better to get it from the opticians or others 

 who keep it. It requires heat in the application, as will be explained 

 in Chapter IV. ; but it is soluble in few, if any, liquids used by the 

 microscopist, and for that reason is serviceable in the manufacture 

 of cells, &c. Where two pieces of glass are to be firmly cemented 

 together, it is almost always employed ; and in all glass troughs, 

 plates with ledges, &c., the beginner may find examples of its use. 



