MIRRORS AND LOOKING GLASSES. 241 



miles down stream from a spot where a river becomes contami- 

 nated with sewage or such matters as refuse liquors from 

 tanneries, paper mills, and similar industrial factories, the water 

 is considerably less impure than just below the spot where the 

 contamination occurs. "When the water is naturally of a slightly 

 ferruginous character, or where clay, &c., containing iron is dis- 

 seminated through the water rendering it turbid, this natural 

 purification seems to take place more rapidly, owing to the oxygen 

 carrying action of the ferruginous matter. 



CHAPTER XVII. 

 MUTUAL ADHESION OF SOLIDS AND LIQUIDS. 



It is an everyday observation that when the hand, stones, and 

 most other solid objects are placed in water, on taking them out 

 again the water wets their surface or adheres to them ; just the 

 same thing occurs with spirits of wine or olive oil ; but, on the 

 other hand, if a solid object be greased with a film of oil and then 

 immersed in water, it will not be wetted therewith when taken 

 out again; whilst, conversely, a thoroughly wet finger may be 

 placed in oil without becoming greased in the same way that a 

 dry one would. This arises from the very feeble adhesion between 

 water and oil as compared with the adhesion of either to a solid. 

 On dipping the hand into a basin of clean quicksilver and remov- 

 ing it, no adhesion will be noticed ; the same remark applies to a 

 piece of glass if the mercury be pure ; but if it contain other 

 metals dissolved therein, it will sometimes adhere to the glass, 

 forming a shining metallic coating. One method of coating the 

 inside of globes and other hollow glass vessels with a reflecting 

 layer of bright metal so as to form mirrors is based upon this 

 property. 



Expt. 288. To Silver a Looking Glass. The ordinary mirrors 

 or looking glasses used for furniture, &c., consist of a flat sheet 

 of glass to which is applied a coating of tin and quicksilver in such 

 a way that the brilliant surface of the metal is preserved from 

 oxidation or injury by the hinder surface of the glass plate. To 

 silver a large sheet of glass properly requires much practice and 

 dexterity ; but a small piece may be coated without much diffi- 

 culty. The glass must be scrupulously clean and recently polished 



Q 



