340 The Microscope. 



one ring should leave sections, not less than ts inch long intact 

 between each incision, and so hard that no impression can be 

 made on the ring with the edge of a stout thumb nail. This I 

 consider a reliable test. But if large sections of the ring fly 

 away, leaving no trace of the cement ou the glass when tested 

 in this manner, as do all the white zinc cements that I have 

 bought of the opticians, it is, without any exceptions, a nuisance. 



It is impossible to prepare a good and reliable cement or var- 

 nish out of poor or improper materials ; nor is it always pessible 

 to i^repare a good article out of the best material, if improperly 

 proportioned, or if prepared in a hasty and careless manner. 



When the best glue or gelatine is soaked in cold water all 

 night and then boiied in a water bath till thin and thoroughl)^ 

 dissolved by frequent stirring and by adding to it prepared 

 chalk, chloride of sodium, glycerine (C. P,) and acetic acid in 

 the proper proportions, we have a reliable cement for mounts 

 and labels that will never crack nor scale off, 



I have found the following original formula good ; but it ma}' 

 be improved, I think : 



1, Reduce 6 drams of dry gelatine to a thin solution in dis- 

 tilled water — soaked over night cold, and boiled in a water-bath. 

 2. Reduce 1 dram of prepared chalk to a thin solution in distilled 

 water, and add to it 1 fluid dram of a strong solution of chloride 

 of sodium (half that quantitj^ of strong alum solution, or a lit- 

 tle chloride of calcium may be better,) and stir it well ; then 

 pour it quickly into the gelatine solution, and stir the whole 

 thoroughly, boiling it until as thick as can be poured into a 

 bottle with a large neck. 3. Mix 1 fluid-dram of alcohol, 95 per 

 cent., into a fluid dram of sulphuric ether ; pour into the gelatine 

 and mix well. If too thick, thin it with acetic acid to suit, and 

 add G or 7 drops of glycerine, (C. P.;) mix the whole thoroughly 

 with a clean stick. 



Do not insert the cork and shake the bottle with the ether 

 and alcohol in it, or it will generate suflicient vapor to burst the 

 glass or blow the cork out with half the cement. When it gets 

 too thick in a warm room through evaporation, thin with alco- 

 holic ether if you want it to dry faster, or with acetic acid to 

 dry slowly. 



Try it on glass, and if too brittle and liable to crack, add 

 more glycerine, say two drops at a time, mixing well and repeat 



