286 SUMMARY OF CURRENT RESEARCHES RELATING TO 



may be rubbed, if it is immediately held over a flame, a certain 

 amount of moisture will appear.* 



" The evaporation from certain kinds of cement, without doubt 

 aggravates the difficulty, and probably this is, in some cases, the 

 independent cause of ' sweating.' 



" Nearly all of the slides examined were prepared in the following 

 way : First, the cover-glass being held in position upon the slide by 

 a clip, the moisture was expelled by heating. After the glass had 

 become sufficiently cooled, small bits of white wax were placed around 

 the edge of the cover-glass. The blunt point of a heated piece of 

 metal was then passed slowly around the cover, and the melted wax 

 flowed under it, far enough to hold it in position. The larger 

 number of the slides prepared in this way were found to be well 

 preserved. When, however, rings of cement were turned tipon the 

 slides, the protection was in almost every case less perfect. In every 

 case in which shellac with anilin colouring was used, condensations 

 on the under side of the cover-glass were found. The covers of 

 several slides were removed, and in no case was there any sweating 

 found upon the surface of the slide. 



"About eighteen months ago, my attention was called to the use 

 of sheet guttapercha rings for dry mounting. My first experience 

 with these rings was not altogether satisfactory. It is now evident 

 that I did not, at first, apply sufficient heat to expel all of the 

 moisture between the cover and the slide. 



" After an experience of several months, I am convinced that 

 slides prepared in the following way, will remain in a perfect state of 

 preservation for any length of time. Use guttapercha rings having 

 a thickness of about one five-hundredth of an inch, and a diameter 

 about one-twentieth of an inch less than that of the cover-glass. 

 Hold the cover in position upon the ring with a light clip, while the 

 guttapercha is being melted by a gentle heat. If too much heat is 

 applied at first, the ring will lose its normal shape. After the gutta- 

 percha is thoroughly melted, the slide should be heated sufficiently 

 to expel every particle of moisture from under the cover. While the 

 slide is hot apply white wax to the surface, the melted wax will run 

 under the cover and will be stopped by the ring. After covering, 

 the wax can be removed from the surface of the glass with turpentine. 



" I shall esteem it a favour to be informed of any case in which a 

 ruled plate, mounted in this way, has failed to remain in good 

 condition." 



Aeration of Aquaria — Mr. J. W. Stephenson points out that it 

 is impracticable to effectually aerate an aquarium in the way suggested 

 by M. Kiinckel d'Herculais, ante, p. 131. The only really effectual 

 method is to direct a very fine stream of water at a high velocity 

 obliquely upon the surface of the aquarium at about the distance of 

 an iuch. By this means air in the finest possible state of subdivision 

 is carried some distance below the surface with the result of ensuring 

 a thorough aeration of the whole contents. 



* But will not moisture always appear on glass placed over a candle or other 

 flame, through water being formed by the union of hydrogen with the oxygen of 

 the air ? — Ed. 



