L. TECHNOLOGY. 509 



added, the two substances being lirst rubbed up with one 

 hundred and twenty to one hundred and fifty grains of oil of 

 lavender, and then united as speedily as possible with the 

 platinum liquid, after which it is ready for use, as mentioned. 

 The coating of oil mixture thus applied is allowed to dry 

 gradually, and the glass plate thus prepared is then to be in- 

 troduced into a muffle of peculiar construction, in which the 

 resinous substances are decomposed and converted into car- 

 bon, without melting or developing any bubbles, the remain- 

 der constituting a perfect platinum surface. Mirrors thus 

 prepared have a high degree of lustre ; and, as the reflection 

 is from the anterior face, it is immaterial what the character 

 of the glass is, provided the surface be perfectly smooth and 

 free from stria?, or, indeed, whether it be transparent at all. 

 This constitutes a great advantage over the ordinary method, 

 where the transparency of the glass is an object of prime im- 

 portance. Glass mirrors of this kind are translucent when 

 held against the light, and may consequently be used to ad- 

 vantage in forming screens for windows of rooms, and in- 

 closed spaces in offices and stores where it is desirable to be 

 able to look out without difficulty, while at the same time 

 concealed from the view of those on the other side. For this, 

 however, it will be necessary that there be no window or other 

 free opening opposite the plate-glass in question. It is said 

 that the cost of platinum sufficient to prepare ten square feet 

 of glass does not exceed twenty cents. 14 C, CXCV., 464. 



FREEZING MIXTURES. 



It is well known that there are certain so-called freezing 

 mixtures which, by their solution in water, tend to produce 

 a greater or less degree of cold, the most familiar illustration 

 of the fact being seen in the application of salt to ice in freez- 

 ing ice-cream or cooling Champagne the ice melting, but the 

 saline liquid indicating a temperature much below that of 

 frozen water. There are other substances, however, the use 

 of which produces a much greater degree of cold than that 

 obtained by means of salt, the most conspicuous among these 

 being finely pulverized crystallized nitrate of ammonia. If 

 this be dissolved in an equal weight of cold water at 50 

 Fahr., a reduction of temperature to 3.20 Fahr. will result. 



Again, if a mixture of seven parts of sal ammoniac, seven 



