596 



KNOWLEDGE & SCIENTIFIC NEWS. 



[November, 1906. 



Conducted by F. Shillington Scales, b.a., f.r.m.s. 



The Use of the Microscope in Chemistry. 



The microscope is now the indispensable servant of so 

 many branches of science that it is always a matter of 

 wonder to me that it is so little used in chemistry. 

 This is probably due to want of knowledge as to the 

 capacities of the instrument, owing to the fact that the 

 use of the microscope as such, and more especially as 

 an instrument of precision, is practically untaught in 

 our science and medical schools; though most workers 

 with the microscope would listen incredulously were 

 it hinted that they had anything to learn in such mat- 

 ters, their belief being that it is merely necessary to 

 focus the objective and make a rough adjustment with 

 the mirror, to get the best results of which the instru- 

 ment is capable. However this may be, I believe that 

 its aid would be of the utmost service to chemists in 

 many ways, were it more generally used, but more 

 especially, perhaps, in regard to qualitative analyses, 

 where the amount of material available is minute. Dr. 

 W'ormley, in his " Micro-Chemistry of Poisons," states 

 that by micro-chemical reactions, the one-hundred- 

 thousandth of a grain of hydrocyanic acid, mercury, 

 or arsenic, can be readily detected with only a few 

 minutes' labour, and Behrens speaks of the detection of 

 0,006." gr. (0.000006 mgr.) of sulphur or bromine, 

 0.0014 ^gr. of silicon, and 000126// gr. of magnesium. 

 Minute as these quantities are, they are, of course, less 

 delicate than some of the chemical reactions known to 

 us, as in the ammonia process for the analysis of water, 

 which can detect one part of albumen in ten million 

 parts of water, or in spectroscopic analysis, which, ac- 

 cording to Bunsen, is capable of detecting the three- 

 thousand-millionth of a grain of sodium in the air. 

 I believe, however, that further experience will show- 

 that micro-chemical reactions are themselves, some- 

 times, not only speedier, but more delicate than the 

 older and better known methods, and there are many 

 cases, where an exceedingly minute portion of sub- 

 stance is to be examined, in which this form of analysis 

 is the only possible means of satisfactorily ascertaining 

 its nature. For instance, I had myself, not long ago to 

 investigate the contents of some sacs in a microscopic 

 larva, upon which depended a rather important point of 

 development. -Such work would have been impossible 

 by ordinary analytical methods, but though the whole 

 process was carried on by polarized light and observed 

 through a one-eighth-inch objective, the analysis itself 

 was quite simple and straightforward, and resulted in 

 unmistakably showing that the contents of the sacs 

 were uric acid. 



There is no doubt that methods of micro-chemical 

 analysis need special initial training in the use of the 

 microscope, and some preliminary patience until the 

 slight difficulty of using an unfamiliar instrument in an 

 unfamiliar way is mastered. They also need judgment 

 as to- w'hen the microscope is likely to prove of service, 

 and as to what may be fairly expected of it, and they 

 need a natural neatness and orderliness in the worker, 



and some precautions with regard to the instrument 

 and objectives. Prisms for polarized light arc 

 necessary, and the stage of the microscope should be 

 of vulcanite or glass, whilst the objectives should be 

 good, but inexpensive, and can be protected by attach- 

 ing a small piece of broken cover-glass to- the front lens 

 of the objective by means of a little glycerine. Of 

 course, the microscope may with advantage have many 

 other refinements, such as those necessary for measur- 

 ing crystallographic angles and axes, but these require 

 special knowledge to utilise properly. 



Preparing and Mounting Wood Sections. 



The mounting of wood sections is hv no means a 

 dillicult matter, but attention must be paid to several 

 more or less important details. For the cutting of the 

 necessary sections, the knife must be of the finest steel 

 and with the finest edge, and the microtome must be 

 satisfactory also. I have found the ordinary English 

 section-knife or razor insufficiently strong for this pur- 

 pose, and a properly-sharpened plane iron, mounted be- 

 tween two pieces of wood to serve as a handle, is far 

 better. The wood may need softening bv boiling in 

 water for some hours, or by soaking for some days. 

 Hard paraffin must be used for embedding, as it holds 

 the object more firmly and shrinks less, and though it 

 will probabh' roll a little, a light pressure with the 

 ball of the finger will rectify this. The thickness of the 

 sections is a matter of some importance, and the 

 general tendency is to cut them too thin. If tran.s- 

 parency is required, the sections must be bleached, but 

 this must be done with care, as over-bleaching des- 

 troys and disintegrates the fibres of the section, whilst 

 insufficient bleaching gives a blotchy appearance. In 

 most cases, it is sufficient to bleach until the colour is 

 discharged from the wood, but no longer, and to follow 

 with a very thorough washing in many changes of 

 water. In this connection an " anti-chlor," such as 

 hypo-sulphite of soda, might be used with advantage, 

 but in any case the final washing must not be curtailed. 

 A good bleaching solution, and one which is not too 

 drastic, is chlorinated soda, made according to the in- 

 structions given by Mr. Cole, as follows : Dry chloride 

 of lime, 2 ozs.; washing soda, 4 ozs.; and distilled 

 water, 2 pints. Mix the lime in one pint of the water 

 and dissolve the soda in the other. Mix the two solu- 

 tions together, shake well, and let the mixture stand 

 for twenty-four hours. Decant off the clear fluid, filter, 

 and keep in a stoppered bottle in a dark place (to pre- 

 vent dissociation), or cover the bottle with paper. If 

 it is required to stain the sections, Delafield's 

 ha:matoxylin, Bismarck brown, or, for double staining, 

 Grenacher's borax carmine and methyl, or aniline green, 

 are f)erhaps the most generally useful. The hsematoxy- 

 lin is frequently made in too strong a solution. It is 

 best to stain slowly in a comparatively weak solution, 

 and when using hsematoxylin, it is a good plan to wash 

 finally in hard water from a tap, as the lime salts have 

 a tendency not only to deepen, but to fix the colour. 

 The Bismarck brown is useful for very delicate struc- 

 tures, or for large spiral or scalariform vessels. Tlie 

 double staining is best carried out by immersing the 

 section in borax carmine for twelve hours or more, 

 washing quickly, but well, in fifty per cent, alcoho!, 

 placing for two or three seconds only in aniline or 

 methyl green, washing as before, and then again stain- 

 ing in borax carmine until the red appears, changing 

 the supply of stain after the superfluous green is dri\en 

 out. It has been recommended to finallv mordant the 



