422 



sulphuric acid dissolves the albuminous matters in the cold, and yields 

 a nearly colourless solution, without altering the nature of the albu- 

 men. But if the liquor be heated to 160°, it acquires an extremely 

 beautiful rich crimson hue, during which change the nature of the 

 albumen is totally altered. Muriatic acid acts upon albuminous 

 matter nearly in the same manner ; but the colour which the solution 

 acquires by being heated is a very rich purple. The jelly from the 

 Leucanthemum affords no crimson colour with sulphuric acid. After 

 pointing out other circumstances of difference, the author gave his 

 opinion that the Leucanthemine ought to be considered as a distinct 

 substance sui generis, another vegetable principle; and concluded 

 Avith expressing his hope that the Camellia, Magnolia, and Chrysan- 

 themum furnish three to the long list of vegetable proximate prin- 

 ciples. 



2 On certain Negative Actions of Light. By Professor Moser 

 of Koenigsberg. In a Letter to Sir David Brewster. 



" There are certain actions of light which may be called negative 

 actions. By this name I mean to indicate a series of new actions, 

 without pretending to enter into any theoretical considerations. In 

 acting upon an iodised plate, common light produces successively 

 different states, which are made manifest by exposing the plate, when 

 taken out of the camera, to the vapours of mercury. If the plate 

 has remained in the camera but a short time, we shall see only the 

 first traces of an image. The parts most strongly affected by the 

 light condense the mercurial vapour only in small quantities. It 

 the light acts daring a longer time, the image will appear with more 

 details and more distinctly, the mercurial vapours being condensed 

 more abundantly. By increasing the time of action, the image gains 

 in details, but the parts which have been affected with a strong light 

 condense the mercurial vapours in smaller quantities. The images 

 that exhibit many details have usually at the same time a grey 

 aspect, arising from the mercurial vapours having already begun to 

 blacken the iodised silver. If the plate remains still longer in the 

 camera, it does not present any visible image ; but when exposed to 

 the mercurial vapours, there is produced an image wholly negative. 

 The parts which have been too long acted upon by an intense light 

 condense little vapour, and hence become completely black, while the 

 parts which were in shadow condense much vapour, and appear ivhitc. 

 Hence arises the peculiar aspect of the image. When this period of 





