CLEANING THIN GLASS COVEES. 217 



fine soft powder, such as starch. Before using it, however, the 

 Microscopist should be careful to clean it thoroughly ; not merely 

 for the sake of removing foulnesses which would interfere with 

 the view of the object, but also for the sake of getting rid of 

 adherent starch-grains, the presence of which might lead to 

 wrong conclusions, and also of freeing the surface from that 

 slight greasiness, which, by preventing it from being readily 

 wetted by water, frequently occasions great inconvenience in the 

 mounting of objects in fluid. The thicker pieces may be washed 

 and wiped without much danger of fracture, if due care be em- 

 ployed ; but the thinner require much precaution ; and in cleans- 

 ing these, the simple method devised by Mr. Spencer will be 

 found very useful. This consists in the use of a pair of round 

 flat disks, about 1J inch in diameter, made of wood or metal 

 covered with chamois leather, and furnished with handles ; for 

 when a piece even of the thinnest glass is laid upon one of these, 

 it may be rubbed clean with the other, and any amount of pres- 

 sure may be used, without the least risk of breaking it. Pre- 

 viously to doing this, however, it will be advantageous to soak 

 the pieces for a time in strong sulphuric acid, and then to wash 

 them in two or three waters ; if greasiness, however, be their 

 chief fault, they should be soaked in a strong infusion of nut- 

 galls ; with which it will be also advantageous to cleanse the 

 surface of glass slides that are to be used for mounting objects 

 in liquid. 



120. Varnishes and Cements. There are three very distinct 

 purposes, for which cements that possess the power of holding 

 firmly to glass, and of resisting, not merely water, but other pre- 

 servative liquids, are required by the Microscopist ; these being 

 (1) the attachment of the glass covers to the slides or cells con- 

 taining the object, (2) the formation of thin cells of cement only, 

 and (3) the attachment of the glass plate or tube-cells to the 

 slides. The two former of these purposes are answered by liquid 

 cements or varnishes, which may be applied without heat ; the 

 last requires a solid cement of greater tenacity, which can only 

 be used in the melted state. The Varnishes used for mounting 

 objects in liquid, should always be such as contain no mixture of 

 solid particles. This is a principle on which the Author, from an 

 experience of many years, is disposed to lay great stress ; having 

 often made trial, at the recommendation of friends, of varnishes 

 which were said to have been greatly improved by thickening 

 with litharge or lamp-black ; and having always found that, al- 

 though they might stand well for a few weeks or months, they 

 became porous after a greater lapse of time, allowing the evapo- 

 ration of the liquid and the admission of air. He has himself 

 found none more durable than that known as japanner's G-old- 

 size, which may be obtained at almost every color-shop ; for al- 

 though this, when newly made, is apt to be somewhat too thin, 

 so as to tend to run in beneath the glass cover, it may be easily 



