Metals and other I todies on a Photographic Plate. 103 



with the pale drying oil which is prepared by Messrs. Winsor 

 and Newton. Pure turpentine is also a very active substance, and, 

 owing to its volatility, in many experiments very useful. These 

 bodies can be used either as liquids in small dishes or by saturat- 

 ing Bristol board or other neutral and porous bodies, such as ignited 

 pumice stone, &c., with them. In the case of the drying oil it can 

 be painted on glass or cardboard and allowed to harden. 



The experiments described in the former communication have been 

 repeated under the same and under slightly different conditions and 

 m another laboratory ; the results obtained with one exception con- 

 firm the previous statements. Glass, selenite, mica, even in very 

 thin layers, are absolutely opaque to the action, whereas gelatin, 

 celluloid, collodion, guttapercha tissue, tracing paper, parchment, 

 and paper are more or less transparent. Linseed oil and turpentine 

 may fairly be taken as typical examples of active organic bodies. 

 This property of acting on the photographic plate is far from 

 belonging to all volatile organic bodies ; for instance, although vege- 

 table oils have the power of acting, mineral oils, so far as they have 

 been tried, have not the power. Benzene, carbon disulphide, chloro- 

 form, &c., also are without this power of acting on the photographic 

 plate ; but the question of what substances are and what are not 

 active will be dealt with on another occasion ; at present it is simply 

 to consider the conditions under which linseed oil and turpentine act 

 on the photographic plate. The picture copal varnish which was 

 much used in the former experiments obviously owes its activity to 

 the turpentine and the oil it contains. Warm the varnish for some 

 length of time, these bodies are driven off and an inactive gum 

 remains. This experiment obviously suggests a vapour as the cause 

 of the action ; at the same time would such a vapour pass through 

 bodies such as gelatin, celluloid, &c. ? With regard, for instance, to 

 the thin-sheet gelatin, it appears to offer but very slight obstruction 

 to the action of these organic bodies; if the thickness of the gelatin 

 be increased still the action takes place, only the time of exposure 

 has to be considerably longer.* Another striking fact with regard 

 to this emanation from these active bodies is that it gives an accu- 

 rate picture of the surface from which it has come. A hard copal 

 surface on glass will give a picture showing every brush mark, 

 unevenness, and scratch on the surface, and if the action take place 

 through a thin sheet of gelatin, or even as many as six or more 

 sheets, still the picture of the scratches is distinct. The following 

 experiments are apparently strong evidence that the action on the 

 photographic plate is due to the vapour given oif from these organic 

 bodies. 



A piece of Bristol board saturated with drying oil, or a piece of 

 * The thin gelatin is 02 mm. and the thick O'lfi mm. in thickness. 



i 3 



