Polytechnic Association Proceedings. 559 



in the want of a standard of reference; hence observers have differed 

 greatly in their estimates. The metrochrome of Mr. Kiucaid con- 

 tains a rotating drum with six equidistant apertures; three of 

 these transmit the uoimai light of the lantern, and the remaining 

 three are constructed so as to admit flat-sided bottles containing: 

 differently colored chemical solutions. By wholly or partially 

 covering one or more of the latter apertures, and giving a rapid' 

 rotation to the drum, it will be possible to produce a light which, 

 thrown into the telescope, forms the image of an artificial stai-; this 

 may be varied until it exactly resembles the light of the star under 

 examination. It has been proposed to insert colored glasses or 

 precious stones in the rotating drum. Mr. Kincaid, however, pre- 

 fers the use of chemical solutions. 



HYDROGEN IN METEORIC IRON. 



Mr. Graham, of London, the well-known chemist, has been 

 lately experimenting with red-hot and melted metals, to ascertain 

 their power of absorbing gases. Iron made red-hot by charcoal, 

 absorbs carbonic oxide, carbonic acid gas, and nitrogen; the two 

 first bemg the products of combustion, and the latter coming from 

 the atmosphere. The gases thus absorbed may, on reheating the 

 metal, be driven off and examined. On submitting a piece of 

 Lenarto meteoric iron to the action of heat, he found that 86 per 

 cent of the gas evolved was hydrogen, its volume being nearly 

 three times that of the fragment of iron from which it was collected. 

 Mr. Graham has found it difficult to impregnate iron, under the 

 ordinary atmospheric pressure, with more than one volume of 

 hj-drogen; hence he draws the important inference that the mete- 

 oric iron came from an atmosphere composed wholly or mostly of 

 hydrogen; which atmosphere, being very dense, cannot be con- 

 nected with the light cometary matter moving within the limits of 

 our solar system. 



PHOTOGRAPHING ENGRAVERS' BLOCKS. 



It is well known that a picture obtained by the camera is a 

 reverse impression; if this is transferred to an engraver's wooden 

 block, the result will be a right impression, which is the reverse 

 of what is wanted. Engravers, heretofore, have obviated the 

 difficulty by tracing the photographic impression on the wood. 

 Mr. B. F. Taylor, of Philidelphia, dispenses with this additional 

 work by using a reflector, with his camera at an angle of 45*^, by 

 which he throws a reverse picture dh-ectly on the block to be 



