£20 
Dr. Kidd's experiments 
Of the four several substances which result from the dis- 
tillation of the black liquid described in the former part of 
this paper, it is probable that the water and the yellow farina 
are the only real products, and that the others are mere 
educts of that distillation : for, with respect to the water, its 
proportion is variable according to the greater or less degree 
of rapidity with which the distillation is conducted ; and if it 
were present as water in the black liquid, there is reason to 
believe it would be found supernatant on its surface, after 
having remained still for some time. The essential liquid 
oil, and the white concrete substance, which pass over during 
the distillation, are probably contained originally in that thin 
portion of the black liquid which may be filtered through 
unsized paper ; for the odour of this filtered portion closely 
resembles that of the oil ; and the oil, by exposure to light, 
frequently becomes of a darker and darker shade, so as at 
last to be nearly of a deep brown colour ; and, with respect 
to the white concrete substance, this was not only found 
crystallized in that part of the original apparatus where the 
black liquid was condensed, but has been obtained from that 
liquid by simple evaporation of it at the common tempera- 
ture of the atmosphere. 
The yellow farina is probably produced from the tar 
which is contained in the proportion of about one-fourth in 
the black liquid ; for it does not make its appearance till 
towards the end of the distillation ; when the more volatile 
substances have ceased to pass over, and the heat has been 
increased to the utmost : and if common coal tar be exposed 
to a low red heat, it will be found, that when the tar has 
