900 Transactions of the American Institute. 



defined crystals, wliich, on analysis, Avere found to contain, in 100 

 parts : Silver, 27.4 ; quicksilver, 72.6 ; and traces of gold. The native 

 amalgam of these two metals contains, in 100 parts : Silver, 36 ; and 

 mercury, 64 ; indicating that one atom of silver is united with two 

 of mercury ; while the crystals above alluded to correspond with the 

 formnla expressing the combination of one atom of silver with three 

 atoms of mercury. 



Use of Zixc-wuite as a Pigment. 

 Dr. Dingier, in his Polytechnic Journal, for September, 1869, dis- 

 approves of the use of any substances containing lead, along with 

 zinc-white (oxyd of zinc), for painting. Instead of the ordinary 

 boiled linseed oil, he recommends an oil prepared as follow : Two hun- 

 dred pounds of linseed oil are gently boiled first, for five or six hours, 

 alone ; and next for twelve hours along with twenty-four pounds of 

 coarsely broken-up peroxyd of manganese; in this manner a very 

 quickly drying oil is obtained, which is eminently fit to be used with 

 zinc-white or any other zinc colors. The oil so prepared should be 

 carefully excluded from the air until it is required for use, in order 

 to prevent its becoming too thick. The zine-white should be first 

 mixed with from three to five per cent of raw oil, after which the 

 prepared oil may l)e added. 



Perception of Color. 



Experiments have recently been made in Germany to ascei'tain tjie 

 relative time required to discern difibrent colois. It was found that 

 the color most easily and quickly distinguished is bright yellow, 

 while those most difticult to recognize are the red and violet. Bright 

 olive comes next after yellow in being readily perceived, and green 

 seems to occupy a middle position. These experiments may have 

 some weight with those who are devising signal lights to guide loco- 

 motion on land and water. Obviously the reasoil why red and violet 

 are not so quickly perceived, is that they are at the opposite extremes 

 of the solar spectrum, and extend res[)ectively to the line where 

 undulations cease to give any impression of light. On the other hand, 

 tlie least sensitive e^-e should be most impressed, as that of the young 

 child is, by the red, because it is produced by the longest undula- 

 tions, which are also those of least velocity. 



Dr. Vanderweyde remarked, that in the primatic spectrum the 

 yellow rays, situated near the middle, are the most luminous, while 



