266 THE FERRIC-OXALATE PHOTOMETER. [MEMOIR XVIII. 



as is easily proved by causing a sunbeam to pass through 

 two parallel strata of the oxalate, when it will be found 

 that the light which has gone through the first portion 

 is inoperative on the second. 



Other properties which this solution of ferric oxalate 

 possesses strongly recommend it as a photometric agent. 

 Unlike solution of chlorine, it may be very conveniently 

 confined in glass tubes by mercury. In its use there are 

 two points which must be attended to: (1) the lemon- 

 yellow ferrous oxalate must not be permitted to incrust 

 the side of the glass exposed to the light, and thereby 

 injure its transparency ; (2) the ferric solution must be 

 kept nearly at a constant temperature, for its color 

 changes with the heat. At the freezing of water it is 

 of an emerald-green tint ; at the boiling, of a brownish- 

 yellow. With these variations of tint its absorptive ac- 

 tion varies, and therefore its liability to be changed. In 

 an extensive series of experiments made with it, but 

 which I have not yet published, I found that it is great- 

 ly improved by the addition of an aqueous solution of 

 ferric chloride. 



It may be remarked that the oxalate is an excellent 

 photographic substance. A piece of tissue-paper, made 

 yellow by being dipped into a neutral solution of it, 

 when dried in the dark is very sensitive. Its invisible 

 impression may be developed by a weak solution of 

 nitrate of silver, two grains dissolved in an ounce of 

 water answering very well. A weak solution of chlo- 

 ride of gold is a still more sensitive developer. I have 

 in my possession photographs made by both these meth- 

 ods more than thirty years ago, which have undergone 

 apparently no deterioration. 



In the application of ferric oxalate to photometry sev- 

 eral methods may be followed. The course I have most 

 commonly taken has been to determine the quantity of 



