378 Dr. Karl Heumann's Contributions to 



In the second section of his paper Hirn notices a statement 

 made by Offret*, who, reasoning from Arago's observation 

 that the luminous effect of a flat gas-flame, as measured by 

 the photometer, is the same whether the broad or narrow end 

 be turned towards the instrument, concludes that the luminous 

 gas-flame is completely transparent. Hirn shows from his own 

 observations that Arago's statement is not quite correct, and 

 that the broad side of the flame always emits a little more 

 light (about one fifth more) than the narrow side of the flame. 

 Offret mentions the well-known fact that the light from an 

 oil-lamp or candle, when thrown through the electric or lime 

 light, casts a shadow on the opposite wall : he, and Hirn also, 

 regards this phenomenon as due to refraction caused by the 

 heated, and therefore thin, layers of gas. 



I have, however, shown that luminous flames throw true 

 shadows when .carefully examined, and that these shadows 

 are independent of the darker and lighter bands caused by 

 refraction. 



In section 3 of his paper Hirn examines the transparency 

 of the flame of petroleum-lamps for light emitted from flames 

 of the same kind. An old-fashioned shadow photometer was 

 employed. A system of eight large lamps placed one behind 

 another, served as the source of light. Hirn's observations and 

 calculations led him to the conclusion that the light from such 

 lamps suffers a greater diminution by passing through the hot 

 gaseous products of combustion, than by passing through 

 the luminous flames themselves. The method of calculation 

 adopted by Hirn appears to me to be altogether erroneous. 

 By applying what I must regard as a more rational method 

 of interpretation to Hirn's results, I find that there is a 

 decided decrease in luminosity brought about by the action 

 of the separated carbon upon the light passing through the 

 flames of these lamps, over and above that occasioned by 

 refraction in passing through the heated layers of gas f. 



Hirn discusses the transparency of flames for light from 

 foreign sources in the fourth section of his paper : he again 

 unfortunately chooses the flame of burning phosphorus. 

 From the fact that the flame of burning phosphorus causes 

 no shadow, while the smoke, so soon as it becomes visible to the 

 eye, does cast a shadow, Hirn concludes that the solid phos- 

 phorus pentoxide suspended in the flame is transparent ; we 



* Essay presented to the Societe d' Agriculture, Douai. 



t [The original paper contains Hirn's numbers, with a description of 

 his, and of Heumann's method of calculation : the latter method is cer- 

 tainly much the more rational. Hirn's photometric process is also shown 

 by his own results to be very faulty. — M. M. P. M.] 



