396 Proceedings of the British Association. 



phere which were observed immediately after, would seem to strength- 

 en this opinion. 



Mr. Hunt called attention to a peculiar condition of the tide on the 

 5th of July, at Mount's Bay, Cornwall. The tide had receded for about 

 half an hour, when it was observed to flow again, and continued to do 

 so for about ten minutes, when it again receded. This was three times 

 repeated. It was observed that this took place over an extensive line of 

 coast, from Land's End to Port Leaven. 



Section B. Chemistry and Mineralogy. 



" Chromatype, a new Photographic Process" by Mr. Jt. Hunt. While 

 pursuing an extensive series of researches on the influence of the solar 

 rays on the salts of different metals, Mr. H. was led to the discovery of 

 a process by which positive photographs are very easily produced. Sev- 

 eral of the chromates may be used in this process ; but those of mercu- 

 ry or copper are preferable — the most certain effects being produced by 

 the chromate of copper, and, indeed, in a much shorter time than with 

 any of the other chromates. The papers are thus prepared : good wri- 

 ting paper is washed over with a solution of the sulphate of copper and 

 partially dried ; it is then washed with a solution of the bichromate of 

 potash and dried at a little distance from the fire. Papers thus- prepared 

 may be kept for any length of time, and are always ready for use. They 

 are not sufficiently sensitive for use in the camera obscura, but they are 

 available for every other purpose. An engraving — botanical specimens 

 and the like — being placed upon the paper in a proper photographic 

 copying frame, it is exposed to sunshine for a time, varying with the in- 

 tensity of light from five to fifteen or twenty minutes. The result is 

 generally a negative picture, which, being washed over with a solution of 

 nitrate of silver, a very beautiful deep orange picture upon a light dun 

 color, or sometimes perfectly white ground, is immediately produced. 

 This picture is quickly fixed, by being washed in pure water and dried. 

 If saturated solutions are used, a negative picture is produced ; but if 

 the solutions are diluted with three or four times their bulk of water, 

 the first action of the sun's rays is to darken the paper, and immediately 

 a very rapid bleaching action follows, giving an exceedingly faint posi- 

 tive picture, which is brought out in great delicacy by the nitrate of sil- 

 ver. It is necessary that pure water should be used for the fixing, as 

 the presence of any muriate damages the picture, and hence arises an- 

 other pleasing variation of the chromatype. If the positive picture be 

 placed in a very weak solution of common salt, the images slowly fade 

 out, leaving a very faint negative outline. If it be taken from the so- 

 lution of salt and dried, a positive picture of a lilac color may be pro- 

 duced by a few minutes' exposure to sunshine. Prismatic analysis has 



