170 Dr. Waller's additional Observations on Hail, 



plates slightly modified. Some of these rhombic crystals were 

 separate, about the tenth of an inch in size, and with angles 

 of about 50° and 130°: some of the crystals formed long, 

 thin, narrow blades, above half an inch in length, with a bright 

 metallic lustre: on their edges were two or three indentations 

 indicating the separate crystals. 



Hijdrostdphate of ammonia. — This is to be obtained by bring- 

 ing into contact ammonia and sulphuretted hydrogen in a 

 gaseous state and })erfectly dry at a low temperature. We 

 then observe the formation of a white crystalline deposit on the 

 sides of the vessel in which the gases are received. Any fur- 

 ther directions respecting the mode of obtaining it will be found 

 in Thenard's Chemistry, Berzelius, &c. 



Hydrosulphate of ammonia is so extremely volatile, that a 

 piece of lint wetted with aether and placed in contact with any 

 part of the bottle which contains it, causes the salt to form a 

 deposit at the inner surface beneath. The crystals in these 

 cases are frequently very regular hexagonal stars, like those 

 of snow. A current of water falling on any part of the bottle 

 rapidly produces the same effect. In summer it generally sub- 

 limes to the upper part of the bottle, on account of the greater 

 heat which is communicated to the base of the bottle by con- 

 tact with the shelf on which it is placed. In a clear night the 

 o(:)posiie effect is produced when placed in contact with the 

 ground. But it is particularly when employed as a differential 

 thermoscope that the indications of this salt are interesting. 

 Thus, during a clear night the salt rapidly accumulates towards 

 the points at which the greatest radiation of caloric takes place. 

 A bottle containing this salt placed between two unequal ra- 

 diating surfaces, indicates, by its change of position, the dif- 

 ference in the radiation of these surfaces. In fact wherever 

 the equable distribution of caloric is disturbed in the same 

 direction for some time, the altered position of the hydrosul- 

 phate serves as an indicator of the course it has taken, and 

 the forms of the crystals show to a certain extent the rapidity 

 with which it has been effected, because where the action is 

 rapid the crystals are much smaller and more confused, than 

 when it has been more gradual. It is surprising how small a 

 difference in the radiating power is sufficient to alter the posi- 

 tion of the salt when properly disposed. I will not at pre- 

 sent enlarge any further on the thermoscopic properties of the 

 hydrosulphate. It presents all the conditions corresponding 

 to those of water. During its preparation it is frequently ob- 

 served in the air in a state of microscopic crystals representing 

 snow : the crystals formetl by rapid evaporation are exactly 

 like the hexagonal plates of snow. A more gentle action gives 



