■REPORT OF THE POLYTECHNIC ASSOCIATION. 39 



varies; yet there is notliing provided to meet this, except the 

 surplus ingenuity of the inventor, who bids his customer pray 

 for a whole lamp and use half a one. In No. 5, with two fluids, 

 this evil is doubled. The flames are marked by absence of flick- 

 ering, but are agitated by any vibration communicated to the 

 lamp. 



A very transparent oil was furnished for the single trial of 

 No. 5. I have used oils bought at 18c., 16c., 15c. and 14c. per 

 quart; the last three being probably such mixtures as are now 

 almost alone retailed. They may be distinguished by a purplish 

 eflorescence on the surface. Lamp No. 6 loses much of the power 

 of regulation when volatile oils are used. Where deposits of 

 carbon were formed, especially in No. 4, this was the more observ- 

 able the cheaper the oil. 



There is one point which may be aggravated by the use of the 

 more volatile oils, viz : the explosiveness of the vapor mixed 

 with air. In lamps, as No. 1, where heat is carried into the 

 fluid, on the cooling of the lamp after extinguishing the light, air 

 will enter to take the place of vapor and a more or less intimate 

 mixture of air and vapor will exist inside. I have frequently 

 witnessed the explosion of such a combination obtained with less 

 heat than is transmitted downward by No. 1, and probably by 

 No. 2., when fitted to a metal lamp. But the effect is not violent 

 when there is a free vent. The inventor of No. 5, has informed 

 me that later trials made by him have resulted in greater suo- 

 cess with " heavy coal oils," but I have been afforded no oppor- 

 tunity to verify this. 



The combustion of coal oils involves less destruction of the 

 wick than that of common oils, but the greater capillarity of the 

 former produces so much swelling that a wick which does not fit 

 loosely in its tube, when dry, is sometimes moved with difficulty, 

 A case of unusual charring in lamp No. 4 was obviated by sub- 

 stituting a thinner wick. A few days disuse, even when entirely 

 exposed to the air, does not prevent a wick being used without 

 trimming after a few minutes immersion: but the amount of 

 cleaning which occurs in practice so mucii diminishes the capil- 

 lar}^ power of the wick, that an experiment with a new wick 

 affords no criterion of the value of a lamp. The volatilization of 

 the oils used is irregular, affecting both the form of the flame 

 and the inclination to smoke. The latter has been observed to 

 increase with the increase of lateral surface exposed. The mode 



