CHEMICAL SCIENCE. 237 



of the original is restored without the picture being touched, and the 

 change is produced solely by sell-action. 



It is also stated that the very small quantity of alcohol that has been 

 absorbed speedily evaporates when returned to the ordinary atmos- 

 phere, and the surface remains as clear as if newly varnished. Thus, 

 the main principle of the patent now taken out is the self-acting na- 

 ture of the process, being effected by means of vapor alone. 



The London At/tenccum says respecting this operation, " that its 

 satisfactory working and results may now be openly seen and watched 

 upon several well-known pictures in the National Gallery. The pro- 

 cess seems to have been fairly tried, without any fear of detriment to 

 the pictures, and the result is such as to thoroughly warrant the im- 

 portance of the invention. The pictures which have thus been fresh- 

 ened in the National Gallery, so as to recover their almost pristine 

 brilliancy, are, principally those of Rembrandt. It should be, how- 

 ever, stated, the process is understood to be only available for a res- 

 inous body like mastic varnish, and the fact of solid oil being imper- 

 vious to these fumes is a satisfactory guarantee for the safety of the 

 genuine old pictures, painted in pure oil. How it will fare with the 

 more modern paintings, where mastic varnish was so intimately mixed 

 up with the oil, even in the original groundwork, is a matter of some 

 anxiety, and is likely to be the subject of a still more delicate exper- 

 iment. 



THE CHANGES IN DISCHARGED FIREARMS. 



The changes in discharged firearms have been recently investigated 

 by M. Decker, who states that, whatever may be the construction of 

 the weapon, after its discharge, there is produced in its exterior and 

 interior a modification in its physical and chemical characters varying 

 with the progress of time. Immediately after the discharge there is 

 formed in the interior and exterior of the gun a blackish blue deposit, 

 the age of which maybe estimated by the variations in its composition. 

 The red spots of the gun proceed from the action of the residue of the 

 charge on the metal, for an arm that has not been used does not rust 

 in a moist atmosphere in the same manner as one that has been used. 

 Variations in the quality of the powder, and in the construction of the 

 gun, do not exercise any influence over the chemical character of the 

 deposit resulting from the combustion of the powder. M. Decker 

 states, that he has prosecuted his researches with especial relation to 

 copper cannon and to gun-cotton. Thus, chemical science can detect 

 whether a gun has been fired or not, and, to a certain extent, how long 

 it has been so used. 



ON THE UTILIZATION OF SEWAGE. 



The following interesting remarks on the subject of the utilization of 

 sewage, were submitted to the British Association at its last meeting, by 

 Dr. H. Bird. " The London sewage was something enormous in quan- 

 tity. It was collected in immense reservoirs, and then poured into the 

 river at times when it would be swept out to sea. Thus, the whole of 

 the sewage of London, containing important chemical constituents, was 

 utterly wasted. He had no doubt that they should relieve the basin of 

 the Thames completely of the sewage which fell into it from Chelsea to 

 beloAV London ; but with regard to the utilization of the sewage, they did 



