straw board, &c., were able to act even at i distance on a sensitive photographic plate, producing effects simila m appearance and developed in the same way as plates which hat been acted on by ordinary light. At that time sufficient experi mental evidence had not been obtained to determine the nature o this action, or even to clearly indicate its general character, whethe' m tact the action arose from vapour given off by the active body or whether phosphorescence was produced which acted on

more »

... plate' ^ !h g h t l y volatile as zinc . +6 a 6 ° £lve 0:® ordinary temperatures in a few days! aCt strongly on a Pho gelatin ^ 1^1 S-j°U f. a^e to Pass rapidly through media, such as rn^inv ' uloid' collodlon> seemed difficult to realise, although iv i ;°, « e eaV ler exPeriments appeared to indicate that this was • 11 .? 1 0 n w^^oh took place. Later experiments confirm on the plate^ & vaPour is given off, which is the cause of the action on pip *u °r^ard® bodies, as well as m etals, have been shown to act n a t n l Pn^raPhlc Plat*> aad in endeavouring to ascertain the unrip., v 1S acj lon exPer" im ents w ith organic bodies were first rapid!v nhf ^ a resu^s w bich they yield are more easily and action7wn^tA T d than th°Se w ith the metals, and if their mode of exerted bj th e^e to ls ' 1^'°£ * * Io h M y throw. " S1 * on the action I , n ^oriner communication it was stated in direcfp g/ n. ^ C°pal Varnish are active substances, both when from it i t a Photographic plate and when at a distance was ablp f Ur WaS ^ounc^ that the action which, they exerted and c o t paS8. 'hro" eh difforent media. A lthough printing inks m ain c T^r T 63 " nsiderably in composition, the boded nP s T are. const" '. hence it was easy to determine that activity aa. ^P eotin e were the bodies to w hich they owed their w ar l d t, . theSe b°dieS separately behaved in the same linseed oi lafc " l1 °°Pal varnish. Boiled oil-that is ® substance Wd10h ha3 .been heated with oxide of lead-is an active substance, and most of the following experim ents have been made * ' Rov. Soo. Proe.,' vol. 61, p. 424.