TRANSACTIONS OP THE SECTIONS. 4/ 



Mr. Prideaux exhibited a specimen of a compound of oxide of lead with empy- 

 reumatic oil, produced in the distillation of wood, and forwarded by Mr. A. Tunstall 

 of Neath. This compound, which had some resemblance to diachylon, was en- 

 tirely soluble in boiling water, from which sulphuric acid separated the lead, and 

 the oil floated on the surface. 



On the influence of the Ferrocyanate of Potash on the Iodide of Silver, pro- 

 ducing a highly sensitive Photographic Preparation. By Mr. R. Hunt. 



The author having been engaged in experiments on those varieties of photographic 

 drawings which are formed by the action of the hydriodic salts on the darkened chloride 

 of silver, and, with a view to the removal of the iodide formed by the process, from the 

 paper, was led to observe some peculiar changes produced by the combined influences 

 of light and the ferrocyanate of potash. It was found that the ordinary photographic 

 paper, if allowed to darken in sunshine, and then slightly acted on by any hydriodic 

 salt, and, when dry, washed with a solution of the ferrocyanate of potash, became 

 extremely sensitive to light, changing from a light brown to a full black by a mo- 

 ment's exposure to sunshine. Following out this result, it was discovered that 

 perfectly pure iodide of silver was acted on with even greater rapidity ; and thus it 

 became easy to form an exquisitely sensitive photographic paper. 



The method recommended is the following : — highly-glazed letter-paper is washed 

 over with a solution of one drachm of nitrate of silver to an ounce of distilled water ; 

 it is quickly dried, and a second time washed with the same solution. It is then, 

 when dry, placed for a minute in a solution of two drachms of the hydriodate 

 of potash in six ounces of water, placed on a smooth board, gently washed by 

 allowing some water to flow over it, and dried in the dark at common tempera- 

 tures. Paper thus prepared may be kept for any length of time, and is at any 

 moment rendered far more sensivive than any known photographic preparation, ex- 

 cepting the calotype, which it quite equals, by simply washing it over with a solu- 

 tion formed of one drachm of the ferrocyanate of potash to an ounce of water. 



These papers may be washed with the ferrocyanate, and dried in the dark ; in 

 this dry state they are absolutely insensible, but they may at any moment be ren- 

 dered sensitive by merely washing them with a little cold water. 



Paper thus prepared is rendered quite insensible by being washed over with the 

 above hydriodic solution, and from the photograph thus fixed many copies may be 

 taken. 



The author then described the action of the spectrum on this preparation, and 

 pointed out, that the greatest effect was produced by the least refrangible rays, but 

 that all the rays, excepting the extreme red, acted with considerable energy. The 

 impressed spectrum was in all cases distinctly coloured from end to end ; and it 

 was found that the colours of superposed media left a corresponding tint upon the 

 paper, but unfortunately as the paper dried the colours faded. These results bring 

 nearer the probability of being enabled to produce eventually photographic pictures 

 in their native colours. 



The spectra formed on these papers were all surrounded by a marked space, which 

 was protected from the influence of the dispersed light, exhibiting another proof of 

 the fact noticed before, by Sir John Herschel and the author, that a class of rays, 

 having peculiar negative properties, emanated from the edges of the sun. Some 

 spectra and numerous specimens of these drawings were exhibited. 



On Manures considered as Stimulatits to Vegetation. By Professor Daubent. 



In this paper the author discussed the question as to the sense in which manures 

 may be considered to act as stimulants to plants. 



It is evident, that if the term stimulus be understood in an acceptation similar to 

 that in which it is employed with reference to the animal economy, it ought to be 

 confined to bodies which by their presence assist in promoting the secretion and as- 

 similation of the nutritious substances present, and ought not to include such as them- 

 selves afford materials for secretion. Thus salt and other condiments do not them- 

 selves nourish the animal, but by their presence induce its secreting surfaces to 

 assimilate more readily the juices brought into contact with them. 



Now it becomes a fit subject for inquiry, whether manures operate in the for- 

 mer manner or in the latter, and likewise whether the fact, that certain of them 



