Mr Ritchie on a new Photometer. 139 



zelius*s test of copper before the blowpipe ; or by melting 

 a portion of them in the phosphate of soda and ammonia, 

 when the muriatic acid gas escapes with effervescence, and 

 may be detected by its odour. 



The chief result of these researches is, that all the minerals 

 included under the Phmb pliosphate of Haiiy, and which form 

 the green and brown lead-spar of Werner, are combinations 

 of one atom of the chloride of lead, and three atoms of the 

 sub-phosphate or sub-arseniate of the oxide of lead ; and that 

 the phosphoric and arsenic acids may be substituted for one 

 another in these compounds, or may be present in them to- 

 gether, in variable proportions, without the crystalline form 

 being thereby affected. This peculiarity arises from the iso- 

 morphous nature of the two acids. In all these varieties, the 

 lead which is combined with the chlorine, is to the lead in 

 the sub-arseniate or phosphate in the proportion of 1 to 9- 



The process by which M. Wohler separated the arsenic and 

 phosphoric acids, depends on the conversion of the former 

 into orpiment by the action of sulphuretted hydrogen. To a 

 solution of the mineral in nitric acid, an excess of ammonia 

 i6 added, with which the hydrosulphuret of ammonia is after- 

 wards mixed and digested. The sulphuret of lead is collect- 

 ed on a filtre. The clear solution contains phosphoric acid, 

 together with orpiment held in solution by ammonia, and the 

 sulphuret is obtained by neutralizing the alkali, and expelling 

 any free sulphuretted hydrogen by heat. 



Art. XXX. — On a nezo Photometer, founded on the Prin- 

 ciples of Bouguer* By William Ritchie, A. M. Rector 

 of Tain Academy. 



This instrument consists of a rectangular box A B C D, 

 Plate I. Fig 13, about an inch and a half or two inches 

 square, open at both ends, and blackened within for the pur- 

 pose of absorbing the stray-light. Within the box are placed 

 two rectangular pieces of plane mirror C F, F D, forming a 



■ Abridged from the original paper in the Edinburgh Transactions, 

 vol. x. part ii. 



