Dr Davy on the Use of the Microscope. 89 



Preparatory to entering on a chemical examination, it is a 

 desideratum to become generally acquainted with the nature 

 of the subject of it. Such preliminary information the mi- 

 croscope is admirably fitted to afford. 



It is a disputed point whether those peculiar, and, in many 

 respects, singular birds, — the humming-birds, — feed on in- 

 sects or the sweet juices of flowers ; some naturalists main- 

 taining that they live exclusively on the one, others that they 

 live exclusively on the other. By repeated observation, first 

 microscopical, afterwards chemical, made on the contents of 

 their minute stomachs, I have ascertained that insects are 

 the solid food, and that the sweet juice of the nectary of 

 flowers is the ordinary drink of these birds. The tongue of 

 the humming-bird, projectile and bifid, is peculiarly fitted for 

 taking insects ; and when moist with a honied viscid lure, its 

 power is even increased. In every stomach of this bird that 

 I have examined, I have detected with the microscope parts 

 of insects, and sometimes entire and living ones ; and I have 

 found their remains also in the lower intestine. 



Another instance may be given from the urinary excre- 

 ment of insects. In the majority of cases in which I have 

 examined this excrementitious matter, I have found it to be 

 principally the lithate of ammonia, commonly in the form of 

 granules, little exceeding in size the blood-corpuscles of man. 

 In one instance, however — that of the excrement of a small 

 night-moth, subjecting it first to microscopical examination 

 — there were observed, mixed with the granules, well-formed 

 crystals, resembling those of lithic acid, and which were 

 proved to be of lithic acid by their chemical qualities, testing 

 them chemically, and observing the results with the same 

 power, for they were far too minute even to be seen with the 

 naked eye. 



Soils, sands, marls, afford other good examples of the use 

 of the microscope as a preliminary to chemical examination. 



Amongst the valuable deposits in the island of Barbadoes, 

 distinguished for its extraordinary fertility, which it owes 

 mainly to the nature of its soils, are beds of calcareous marl. 

 Under the microscope, this marl appears to be almost homo- 

 geneous, except that amongst the molecules of which it is 



