POPULAR SCIENCE. 3 



proved that an electric current travelling in any one direc- 

 tion deflected a magnetic needle at right angles to itself, no 

 sort of use for this discovery was at the time apparent. It 

 seemed to be in the list of things popularly called useless. 

 Nevertheless, in time, and, for a discovery to take effect, no 

 long time it culminated in one form of the electric tele- 

 graph. Then, again, when Faraday proved, that by winding 

 an insulated wire around an iron bar, and transmitting elec- 

 tricity through the wire, the bar was instantaneously con- 

 verted into a powerful magnet, he laid the foundation of 

 other varieties of the electric telegraph ; and the electricians 

 who, before his time, proved that electricity could be made 

 to develop colour in a chemical salt furnished a principle on 

 which is based a third variety. Up to this day all the electric 

 telegraphs made or proposed whether to indicate by bell- 

 ringing, dial-work, actual printing in ordinary letters, print- 

 ing by accepted signs, or telling their tale by change of 

 colour (and electric telegraphs can do all this) are but de- 

 velopments of three electrical functions that, when discovered, 

 seemed as far from useful application as well might be. Take 

 -electrotype, again ; the beautiful process whereby a thin layer 

 of gold, silver, or other metal may be deposited on a conduct- 

 ing surface; consider the numerous practical applications of 

 this art, the manifold ways in which it conduces to the 

 utilities of life : the principle on which it depends was known 

 some time before any practical application became apparent. 



A very remarkable application of science, that seemed re- 

 mote from any human need, is even now taking place to most 

 utilitarian purposes. After the decisive proof offered by Dr. 

 Tyndall at a Friday-evening Royal-Institution lecture, that 

 air can be mechanically purified from dust by transmission 

 through a filter of cotton-wool, it is impossible the demon- 

 stration can stop short of effecting a rational means of venti- 

 lation. Hitherto, ventilative contrivances of whatever kind 

 liave merely had the end in view of admitting to apartments 



