72 REPORT—1840, 
surface of diluted hydrochloric acid (one part strong acid to seven or ten 
of water), then quickly washed in water, and slightly and carefully 
dried in a heat not exceeding 212° of Fahr. The paper is in this state 
ready for being bleached by the rays of the sun; and in order to fix 
the obtained drawing, nothing more is required than to steep the paper 
a few minutes in alcohol, which dissolves the free bichloride of 
mercury. The maceration must not be continued too long, as in that 
case the paper begins to darken again. 
The author’s second method of producing positive photogenic draw- 
ings was by using metallic plates, and covering them with a layer of 
hydruret of carbon, prepared by dissolving pitch in alcohol, and col- 
lecting the residuum on a filter. This, when well washed, is spread as 
equally as possible oyer a heated even metallic plate of copper. The 
plate is then carbonized in a close box of cast iron, and, after cooling, 
passed betwixt two polished steel rollers, resembling a common copper- 
plate printing-press. The plate, after this process, is dipped into the 
above-mentioned solution of the nitrate of silver, and instantly exposed 
to the action of the camera. The silver is, by the action of the rays 
of the sun, reduced into a perfect metallic state, the lights are expressed 
by the different density of the milk-white deadened silver, the shadows 
by the black carbonized plate. In a few seconds the picture is finished ; 
and the plate is so sensitive, that the reduction of the silver begins even 
by the light of a candle. For fixing the image, nothing else is required, 
except dipping the plate in alcohol mixed with a small quantity of the 
hyposulphite of soda, or of pure ammonia. 
On Poisons, Contagions, and Miasms. By Professor Liestc. 
Dr. Playfair stated that he had prepared, at the request of the author, 
a statement of Professor Liebig’s new views on the subject of poisons. 
Poisons might be divided into two classes, belonging to the inorga- 
nic and organic kingdoms. Many substances were called inorganic 
poisons which had in reality no claim to be considered as such. Sul- 
phuric, nitric, and muriatic acid, when brought in contact with the 
animal ceconomy, merely destroyed the continuity of the organs, and 
might be compared, in their modus operandi, to the action of a heated 
iron, or a sharp knife. But there are others—and these are the true 
inorganic poisons—which entered into combination with the substance 
of the organs without affecting any visible lesion of them. Thus it is 
known, that when arsenious acid or corrosive sublimate is added to a 
solution of muscular fibre, cellular tissue, or fibrin, these enter into 
combination with them, and become insoluble; when they are intro- 
duced into the animal organism the same circumstance must happen. 
But the bodies formed by the union of such poisons with animal sub- 
stances are incapable of putrefaction; they are incapable, therefore, of 
effecting and suffering changes; in other words, organic life is de- 
stroyed. The high atomic weight of animal substances explains the 
¢ause of such small quantities being requisite for producing deadly 
