336 Dr Davy's Additional Notice on (he 



Heated with nitric acid, it acquired the colour characteristic 

 of lithic acid. The quantity was too minute to test it for 

 ur6a, which, it is probable, formed a part of it. 



Flies. — In the first trial I made of the excrement of one 

 of these insects, I was unable to detect lithic acid, and I in- 

 ferred that its chief ingredient might be urea. Since then I 

 have examined the excrementitious matter of three different 

 species of fly, — one smaller than the common house-fly of 

 England ; one larger ; and one a carrion-fly, very like the 

 English flesh-fly. In the excrement of each I have detected 

 distinctly lithic acid. In one instance crystals of this acid 

 were observed under the microscope, — crystals of a rhom- 

 boidal form, which, mixed and heated with nitric acid, im- 

 parted the peculiar purple tint of lithate of ammonia. In the 

 same, a single crystal was seen, — a low four-sided pyramid, 

 which, it may be conjectured, consisted of ammoniaco-mag- 

 nesian phosphate. Commonly the proportion of lithic acid 

 was very minute, and much care was required in the manage- 

 ment of the temperature to detect it, in testing it with nitric 

 acid. The secretion, in the three instances, was more or less 

 liquid, and appeared to contain urea, in accordance vyrith my 

 earlier trials. 



Dragon Fly. — One of a large size, voided, in confinement, 

 pretty much reddish excrement. This, under the microscope, 

 appeared to consist of little aggregate masses, with which 

 were intermixed some very thin colourless six-sided plates, 

 not unlike those of the cystic oxide. The principal ingredient 

 of this excrementitious matter, it may be inferred, was lithic 

 acid, judging from the strong purple tint acquired when it 

 was heated with nitric acid. 



Muskitoe. — So great is the delicacy of the nitric acid test 

 for lithic acid, that I thought it probable it might be detected 

 even in the excrement of this minute insect, if it existed in 

 it ; and the result obtained on trial has shewn that the con- 

 jecture was right. In several instances in which the excre- 

 ment of the muskitoe was obtained by confining the insect 

 under a glass vessel scrupulously cleaned, a distinct trace of 

 lithic acid was found in the speck of matter voided by it. One 

 insect, the urinary secretion of which so tested, was found to 



