OF MICROSCOPIC OBJECTS. 59 



is quite as safely performed (if not more so) by making a solution 

 of balsam in turpentine of the consistency of thick varnish. The 

 thin glass cover may be slightly coated with this, and will then be 

 much less liable to imprison any air, which frequently happens 

 when the cover is dry. Bubbles, however, will sometimes make 

 their appearance in spite of all care ; but when the object is com- 

 paratively strong, they may be removed by keeping the slide ra- 

 ther warm, and working the cover a little, so as to press them to 

 one side, when they should be immediately removed with a needle 

 point, otherwise they are again drawn under. 



Where the slide requires keeping warm for any length of time, 

 a hot-water bath is sometimes made use of, which is simply a flat 

 tin, or other metal case, with a mouth at the side, that when the 

 hot water is introduced it may be closed up, and so retain its 

 warmth for a long time. In working, the slide is laid upon it, 

 and so admits of longer operations, when required, without grow- 

 ing cold. Sometimes a spirit-lamp is placed under it to keep up 

 an equal heat through excessively long processes. Where the 

 time required, however, is but short, a thick brass plate is some- 

 times used (see Chapter I.), which is heated to any degree that 

 is required, and the slide placed upon it. 



Some objects, which -are so thin that they are usually floated 

 upon the slide, as before stated, require no steeping in turpentine 

 or other liquid. These are best mounted by covering with a lit- 

 tle diluted balsam, and after this has had time to penetrate the 

 substance, ordinary balsam is laid upon it, and the slide finished 

 in the usual manner. 



I have stated that the balsam is usually applied to the slide and 

 objects with a " bluntly-pointed glass rod ;" but for the purpose of 

 drawing the balsam from the bottle, and conveying it to the de- 

 sired place, Dr. Carpenter uses a glass syringe with a free opening. 

 These are his instructions : " This (the syringe) is most readily 

 filled with balsam, in the first instance, by drawing out the pis- 

 ton, and pouring in balsam previously rendered more liquid by 

 gentb warmth ; and nothing else is required to enable the opera- 

 tor at any time to expel precisely the amount of balsam he may 

 require, than to warm the point of the syringe, if the balsam 

 should have hardened in it, and to apply a very gentle heat to the 



