242 SCIENTIFIC AMUSEMENTS. 



with a clean leather, &c. ; any grease or other matter adhering to 

 the glass will inevitably cause blemishes. Spread out on a flat 

 plain board placed on a table a piece of thick tinfoil about the 

 size of the glass plate, and smooth it with the finger ; pour on the 

 centre a few drops of mercury and rub these very gently with a 

 hare's foot, or pad covered with flannel, so as to make the mercury 

 wet the tinfoil; gradually add more mercury and extend the 

 rubbing outwards until finally the whole of the tinfoil is coated 

 with quicksilver. Pour on more mercury until the liquid metal 

 forms a coating of the thickness of a shilling. Now gradually 

 slide the glass plate polished side downwards over the tinfoil, so 

 as to push most of the mercury in front of it without allowing 

 any air bubbles to enter between the glass and metal ; if this is 

 dexterously done, the under surface of the glass and the mercurial- 

 ised tinfoil will form a brilliant mirror ; but any unsteadiness of 

 the hand in pushing the glass plate will cause particles of scum 

 and dust or air bubbles to get between the glass and metal, causing 

 spots and blemishes. Much of the mercury originally poured on 

 is thus pushed off; more runs off when the board is cautiously 

 lifted on one side so as to tilt the mirror and allow the surplus 

 quicksilver to drain away. After remaining half an hour slightly 

 tilted, the mirror should be placed upright to drain still further ; 

 next day the tinfoil will have become entirely penetrated by the 

 mercury that has not drained away, and the tin and mercury 

 together will have set to a solid amalgam, which adheres pretty 

 firmly to the glass, but is easily scraped off, being very brittle. 

 The mirror then only requires to be set in a frame with a solid 

 back to avoid the scratching off of the amalgam during use. 



Processes by which films of pure silver are deposited on glass 

 by chemical action as reflecting agents instead of amalgam have 

 largely come into use of late years, especially for hollow globes 

 and curved surfaces. Expt. 198 illustrates in a rough way how 

 this is effected. 



Before these processes were invented such mirror surfaces could 

 only be coated by making a mixture of mercury and other metals 

 nearly fluid when warm, but setting solid or nearly so on cooling, 

 and of such a nature that when shaken about inside a clean glass 

 globe, &c., the mixture would adhere to the glass, wetting it as 

 water would; so that a film would permanently stick to the 

 glass, forming a reflecting coating when the rest of the amalgam 

 was poured out. A mixture of mercury 2 parts, lead 1, tin 1, 

 and bismuth 1 part, answers well for this purpose. 



Removal of Substances from Solution by Absorption by 

 means of Solids. Many solids when brought in contact with 



