OF MICROSCOPIC OBJECTS. 23 



It must be used whilst hot, and as long as it retains even slight 

 warmth can be readily moulded into any form. It is often em- 

 ployed in making shallow cells for liquids, as before mentioned. 



GUM- WATER is an article which nobody should ever be with- 

 out; but labels, or indeed any substance,, affixed to glass with 

 common gum, are so liable to leave it spontaneously, especi- 

 ally when kept very dry, that I have lately added five or six drops 

 of glycerine to an ounce of the gum solution. This addition has 

 rendered it very trustworthy even on glass, and now I never use 

 it without. This solution cannot be kept long without undergo- 

 ing fermentation, to prevent which the addition of a small quan- 

 tity of an essential oil (as oil of cloves, &c.,) or one-fourth of its 

 volume of alcohol, may be made, which will not interfere in any 

 way with its use. 



There is what is sometimes termed an extra adhesive gum-water, 

 which is made with the addition of isinglass, thus : Dissolve 

 two drachms of isinglass in four ounces of distilled vinegar ; add 

 as much gum arabic as will give it the required consistency. This 

 will keep very well, but is apt to become thinner, when a little 

 more gum may be added. 



All these, except one or two, are liquid, and must be kept in 

 stoppered bottles, or, at least, as free from the action of the air as 

 possible. * 



When any two substances are to be united firmly, I have 

 termed the medium employed " a cement ;" but often the appear- 

 ance of the slides is thought to be improved by drawing a 

 coloured ring upon them, extending partly on the cover and partly 

 on the slide, hiding the junction of the two. The medium used 

 in these cases I term a VARNISH, and hereinafter mention one or 

 two. Of course, the tenacity is not required to be so perfect as 

 in the cements. 



SEALING-WAX VARNISH is prepared by coarsely powdering seal- 

 ing-wax, and adding spirits of wine ; it is then digested at a gen- 

 tle heat to the required thickness. This is very frequently used 

 to " finish " the slides, as before mentioned, and can easily be made 

 of any colour by employing different kinds of sealing-wax ; but is 

 very liable to " chip " and leave the glass. 



BLACK VARNISH is readily prepared by adding a small quantity 



