154 Progress of Invention. 



due to currents in the wing of the animal — the most energetic 

 of which issues from the base of the wing, and follows the middle 

 nervure till it reaches the outer edge. 



Ventilation of Sewers. — M. Robinet, a French chemist, pro- 

 poses to ventilate sewers by causing the air required for combus- 

 tion in factories to be obtained from them. According as the im- 

 pure air is taken away by the draught of the furnaces, pure air will 

 rush in from the atmosphere. He calculates that the Parisian 

 sewers would be emptied of their contents many times a day, even 

 though only some of the furnaces in that city were thus supplied 

 with air. The sewers are a source of great danger in this country, 

 also, from the noxious effluvia they emit, and which but too often find 

 their way even into dwellings. There is no reason why a means of 

 ventilating them so simple and effective should not be adopted. 



Etching in Relief. — M. Boettger has lately invented a simple 

 process for this purpose. The drawing is made with a quill on 

 clean polished zinc, with a solution containing one part dry chloride 

 of platinum, and one part finely powdered gum arabic, in twelve 

 parts, by weight, of water. Before the writing is quite dry, the 

 plate is immersed for a few moments in a solution of auro cyanide of 

 potassium, which covers it over with a thin coating of gold. It is 

 then placed in a liquid containing one part nitric acid, spec. grav. 

 1*2, and sixteen parts water, and rubbed with a camel's-hair brush. 

 The gold peels off, except in the places covered by the lines of the 

 ■drawing • and, after some time, the hollows will have become suf- 

 ficiently deep for the plate to be used in printing. 



Danger from Contact with a Person Struck by Lightning. — 

 It might be supposed that, when any one is struck by lightning, 

 •the electric fluid immediately passes away, on account of the con- 

 ducting power of the animal body, and of the objects in contact 

 with it, especially if moisture is present. This, however, does not 

 always occur; though our present knowledge of the laws of 

 electricity will not suffice to explain the exceptional cases. Two 

 instances illustrating this subject have been brought before the 

 Academy of Sciences by M. Boudin. One occurred on the 30th 

 June, 1854. A man was killed in the Jardin des Flantes at Paris, 

 by lightning, and his body was exposed for some time to heavy 

 rain. When, however, two soldiers attempted to remove it, they 

 received, the instant they touched it, a very violent shock. The 

 other happened on the 8th September, 1858. Two artillery men, 

 at Zara in Dalmatia, were appointed to remove telegraph posts ; 

 on attempting to lay hold of them after a violent thunder-storm, 

 they were thrown down and greatly injured, especially one of 

 them. When a comrade endeavoured to assist him that was most 

 hurt to rise, both were dashed violently to the ground ; the comrade 

 was burned in the arm, and was afterwards affected with nervous 

 symptoms. 



The Sensitizing of Iodide of Silver. — The different deport- 

 ment of iodide, under the action of light, from that of other salts of 



