138 ANNUAL OF SCIENTIFIC DISCOVERY. 



ing their calibre one-tliird, are rendered soluble by the h>-dro- 

 earbon vapor, and run off by the drip as compounds of coal tar. 



A method of increasino; the illuminating power of gas was de- 

 vised some 3"ears ago in England, by Mr, Bowditch, which at the 

 time attracted attention by its claim to superior cheapness and 

 safety. It consisted in placing over the burner a vessel contain- 

 ing naphthaline or other hydrocarbon, vaporizing at a compara- 

 tively high temperature, and connecting the upper part of the 

 vessel with the jet-pipe, so that soon after the ordinary flame was 

 lighted the vapor would be supplied, and mingling with the gas 

 would greatly increase the illumination. The arrangement was 

 perhaps inelegant, especially for dwellings, but appeared to have 

 claims to attention, as one of the processes for economical lighting, 

 now so frequently discussed. 



PLATINIZED MIRRORS. 



The diathermanous properties possessed by various substances 

 are precisely analogous to those of transparency and translucency 

 with which they are endowed, except that the former refer to rays 

 of heat and the latter to those of light. Although in some degree 

 the two descriptions of rays may be confounded, yet they are in 

 reality separate, and the actinic rays of the sun are perfectly dis- 

 tinct from the luminous ones. 



It might be supposed that the substances which showed great 

 power of translucency would also evince simihir capabilities with 

 respect to diathermancy ; but experience has proved this assump- 

 tion to be perfectly erroneous. If we select chloride of sodium 

 in its crude condition, common crystal, and alum, they will be 

 found nearly all equal in their power of transmitting light ; but a 

 wide discrepancy will be found in the manner in which they trans- 

 mit heat. Their diathermanous capabilities are in the proportion 

 of 9, 62, 93. It is quite possible to modify these proportions of 

 bodies so as to produce quite contradictory and almost apparent 

 paradoxical results. Thus a mirror can transmit light, and a per- 

 fectly translucent surface is capable, under certain conditions, of 

 reflecting it. 



By coating upon one side an ordinary plate of glass with 

 an extremely thin layer of platinum, Mr. Dode obtains a mirror 

 with direct reflection, which may also, curiously enough, be em- 

 ployed as a common window-pane by turning the coated surface 

 outside. A slight tinge is imparted to the objects beheld through 

 this medium, but otherwise the vision is clear, aud the outlines of 

 the objects well defined. 



As all rays of light and heat must be disposed of by reflection, 

 absorption, and transmission in different proportions, it is mani- 

 fest that when a transmission and absorption accompany a re- 

 flection, there is a loss incurred when the end in view is to bring 

 into play the reflective powers only of the body. To prevent this, 

 it is the practice to cover the non-platinized surface of the mirror 

 with a slight coating of varnish. In this condition they are, of 



