MEASUREMENT. xxxix 



4. Muriatic acid with heat colours the proteine compounds. (See PROTEINE COM- 

 POUNDS.) 



5. Acetic acid brings into view the nuclei of animal cells and tissues ; dissolves many 

 salts, &c. 



6. Dilute nitric acid (20 per cent.) coagulates albumen, renders unstriped muscular fibre- 

 cells very distinct, &c. Strong acid by boiling removes all but the cellulose from woody 

 fibre. 



7. Millon's test-liquid for proteine compounds. (See MILLON'S TEST.) 



8. dEther dissolves fatty and resinous matters, &c. 



These are perhaps the most common reagents which the experimenter will be called upon 

 to use. A general plan for the qualitative analysis of substances must be obtained from works 

 upon chemical analysis. It may be remarked, however, that the qualitative analysis of 

 portions of a substance too minute to be more than barely discerned by the naked eye, may 

 be effected by the aid of the microscope. The use of the microscope in strictly chemical 

 investigations also, cannot be too highly recommended ; for it will frequently throw great 

 light upon the distinction of chemical precipitates of closely approximative chemical pro- 

 perties. 



D. MEASUREMENT. 



A knowledge of the size of objects is of the utmost importance, and is frequently of great 

 assistance in the distinction of one object from another ; for many objects of totally dis- 

 similar nature present exactly or nearly the same appearances when examined with different 

 powers. The dimensions should invariably be added to the description of microscopic 

 bodies, and when figures are given, the number expressing the linear amplification of the 

 objects should be placed near them. 



A striking instance of the careless manner in which conclusions are formed from micro- 

 scopic appearances, and of the importance of attending to the size of objects, occurred a few 

 years ago, and a sketch of it may serve to impress this importance upon the mind. A 

 diligent microscopic observer discovered a few rounded bodies, the nature of which was 

 unknown, in some human secretions. Another microscopic observer, apparently not 

 acquainted with the method of determining their nature, examined various substances used 

 as food, and in bread detected some of the spores of a fungus, somewhat resembling the 

 former in appearance, although but slightly. The conclusion was at once formed that 

 the two kinds of bodies were of identical nature, and the result was laid before a Society 

 which ought to have been learned in these matters, but no one detected the error. On sub- 

 sequent examination, however, it turned out that the former bodies were about twelve times 

 the size of the latter, and as the smaller bodies were known to be in the mature state, the size 

 alone would have afforded sufficient evidence of their distinct nature. It might, perhaps, 

 be truly stated, that nine-tenths at least of the figures of objects which are published, are 

 of no real use, because their size is not expressed ; hence an observer is incapable of com- 

 paring the appearance presented by any object before him with the figures, or of comparing 

 the results of his investigations with those of others. 



Directions for determining the measurement of objects are given under the head 

 MEASUREMENT. It must always be expressed in fractions of an English inch. 



In conclusion, we must remark, that the observations given in this Introduction are not 

 offered as by any means complete. They will, we hope, however, serve to show those who 

 have not kept their eye for many years upon subjects connected with microscopy, that 

 numerous means are at their command for determining the structure of objects, to indicate 

 the nature of these means, and that microscopic researches should be carried out upon some- 



