Prof. Draper ofi the Rays of Incandescent Lime, ^S5 



position that condensation of vapour produces them, because 

 that process is very irregular in its action ; but if this suppo- 

 sition is admitted to be true, it will follow that the dew-point 

 is not a correct measure of the daily addition that is made to 

 the weight of the atmosphere in the middle of the day by the 

 vapour that has been thrown into it, and therefore it does not 

 present the means of ascertaining the separate gaseous pres- 

 sure. For the same operation that keeps down the dew-point 

 in the middle of the day, creates cloud that floats in and rests 

 upon the whole mass of the atmosphere; and the gaseous 

 portion of that atmosphere must then press on the surface of 

 the earth, not only with its own weight, but with the addi- 

 tional weight of nearly the whole of the cloud that is then 

 floating in it. And if the curve of gaseous pressure, as com- 

 monly given, does not show a rise resulting from this addi- 

 tional pressure, it is because the whole atmosphere is at the 

 same time made lighter by the heat which has been liberated 

 by condeuvsation of vapour. 



LXIX. Account of a remarkable difference het*ween the Rays 

 of Incandescent Lime a?id those emitted by an Electric Spark, 

 By John W. Draper, M.D., Professor of Chemistry in the 

 University oj^ New York*". 



^OME years ago M. Becquerel discovered that the rays of 

 ^ an electric spark, if they were transmitted through a 

 screen of glass, could not excite the phosphorescence of sul- 

 phuret of lime. 



To make this experiment, wash a metallic plate over with 

 gum-water. Dust upon it, from a fine sieve, a quantity of 

 Canton's phosphorus (oyster-shells calcined with sulphur), and 

 allow the plate to dry. An uniform surface is thus obtained, 

 suitable for these purposes. Place before that surface a piece 

 of glass and a piece of polished quartz, and discharge a Ley- 

 den phial a few inches off, so that the rays of its spark may 

 fall on the plate. It will be found that under the quartz the 

 phosphorus will shine as much as on the spaces that have not 

 been covered ; but under the glass it will remain almost en- 

 tirely dark. 



Last winter I observed the curious fact, that when this ex- 

 periment is made with a piece of lime incandescing on a 

 stream of oxygen, directed through the flame of a spirit-lamp, 

 the glass, so far from being unable to transmit the rays, ap- 

 pears to be as transparent, to ; itKeajt as .^qus^rU or atmospherio 



air. -r'Hq ('MJfo ((i hiiP. ••t'th il>j 



•ilfi 4io iit^iidjlCommunieated by the Author. , .)^ 



