74 



KNOWLEDGE 



February. 1')11. 



likely to p;-o\. .i yroup of hitherto unsuspected iniport:ince, 

 both in point of numbers and diversity of forms. .\U these 

 moss-dwellers can revive after desiccation. The adult animals 

 become dormant when depri\ed of moisture and re\i\e when 

 re-moistened. It is not. as ;^:ich,irias concluded from his 

 experiments in 18S5, that the survival of the species is effected 

 by means of eggs. 



Mr. A. Earland ga\e a lantern lecture dealing with the 

 apparatus and meth ids employed in the cruisers of the Inter- 

 national North Sea Commission, with special reference to the 

 Work of the '■ Goldseeker." the cruiser of the Sc'ltti^ll branch 

 of the Commission. 



THE MICiv -^!-^)GIST. — We ha\ r to acknowledge the 

 receipt from the publishers. Messrs. Flatters. Milborne and 

 McKechnie. Ltd., of the third part of their quarterly. It well 

 maintain? the character set in the previous parts and contains 

 full practical details for the staining and mounting of animal 

 and vegetable tissues, such as the pinnae of Ferns and the 

 head of the Crane-fly and Hive-bee. Pages 35-37 are de\oted 

 to the methods adopted for embedding in Celloidin. and the 

 catting and staining of such sections as those of the head of 

 ilie Blow-fly. There are collotype reproductions of photo- 

 micrographs of several of the objects described. Mr. Chas. 

 Turner, F.C.S., contributes an article on Collecting and 

 Preserving Fi^eshwater .Algae. \Ve much regret to see so 

 many errors in the printing cf scientific names, and would 

 suggest to the publishers a more careful revision of the proof 

 sheets. 



N\'e have also received a selection of the slides illustrated 

 and described in Parts 1-3. and can reconnnend them both 

 for c|uality and moderate price. The slides received with 

 Part 3 are: vertical section of the head of the blow-fly, the 

 pinna of Aspidiuin Filix-nias. stained and mounted entire, 

 and the fruiting spike of SchTi;iiicl!a sp., showing macro- 

 and micro-sporangia. 



THE PREPARATION OF A ROCK SECTION.— When 

 commencing the preparation of a rock section, either by liand 

 or with the aid of a machine, the first thing to be done is to 

 procure a suitably-si2ed chip or slice. The si^e I usualh- start 

 with is from three-quarters of an inch to one inch stpiare. 

 This will probably lose a little in grinding, but will be a fairlv 

 large piece when finished. 



If the work is to be done entirely by hand, it will be found 

 possible in most cases to strike off suitably-sized chips with a 

 trimming hammer, or by the aid of a chisel. The thinness of 

 these chips will depend entirely on the texture of the rocks, 

 and while it will be found comparatively easy to get large 

 flakes off fine grained rocks, such as basalts or andesites, it is 

 difficult to get even thick lumps off the coarse granitoid rocks. 

 With the slitting machine, described and illustrated in 

 "Knowledge '" for January, page 30, however, it is easy to get 

 slices about n^nd of an inch, or even less, and it will be seen 

 at once that this saves a lot of time in grinding. 



The method usually recommended for using a slitting disc 

 is to rub the edge with powdered diamond or bort, made into 

 a paste with oil so as to incorporate it in the soft iron to make 

 the cutting edge, but I have found a much easier and cheaper 

 way is to apply a paste of carborundum and water witli a 

 camel-hair brush to the cutting edge just above the rock speci- 

 men. The grade of carborundmn I use for this purpose is 

 No. 150, which cuts fairly fast, and I find by actual timing that 

 I can get through an inch of any ordinary igneous rock in 

 from six to eight minutes without undue exertion. 



When starting to cut a slice with the machine the rock is 

 clamped in the holder in the proper position, so that the piece 

 can be cut in the direction required, and the spindle is then 

 screwed forward so that the rock projects over the cutting 

 edge of the disc. The disc is made to revolve toward the 

 operator, the edge is wetted with the carborundum paste, and 

 the rock is then allowed to touch the wheel. It is advisable 

 to keep the rock against the disc by the aid of the thumb of 

 the left hand so as to prevent it from jumping. The edge of 

 the disc must be kept well supplied with the carboriui(luui and 

 water, as the cutting is then nnich expedited. 



When one piece is cut off, the rock is lifted free of the disc, 

 and is moved forward by the spindle ready for the next slice 

 to be cut. 



Having obtained a suitable piece, either by chipping or 

 cutting, the next thing to be done is to make one side of it 

 perfectly plane and smooth. This can be done by rubbing the 

 rock on a piece of plate glass, say, six to eight inches square, 

 which is moistened with carborundum and water. A zinc or 

 copper plate can be used instead of the .glass, but I have found 

 the glass to cut better and work more quickly. I use the same 

 grade of carborundum for this as I do for slitting, as I find 

 No. 150 is not coarse enough to tear the rock or fine enough 

 to make the work tedious. Coarser grades can, of course, 

 be used, if thought desirable, but it is not advisable to use 

 anything coarser than No. 90. 



With the machine described in the January luunber the rough 

 grinding is done on the lap. I usually make the lap revolve 

 in the opposite direction to the hands of a watch, and hold the 

 rock in the fingers of the left hand against the left side of the 

 lap, so that I push against the revolution of the wheel. This 

 I have found easier than pulling against the wheel. It must 

 be borne in mind that the periphery of the wheel cuts faster 

 than the inner portions, and the section, therefore, must be 

 fretiuenth twisted and moved backward and forward from the 

 edge of the wheel to the centre so as to keep a plane surface. 



When a perfectly flat surface has been procured, the rock 

 must be rubbed by hand on a sheet of glass with finer 

 carborundum, say No. FF, so as to remove all coarse scratches. 

 Care must be taken at this stage not to get any coarse powder 

 on to the glass plate, or it will cause bad scratches on the rock. 

 The latter can be finished off on FF emery cloth, which is 

 supported on a glass plate, and it is then polished on a piece 

 of worn emery cloth. No, 0. It is not necessary to get a 

 high polish with ordinary igneous rock, so long as there are no 

 scratches. 



The next process is to fasten the piece of rock to a suitable 

 handle for the grinding of the reverse side. It is customary 

 to employ pieces of plate glass for this purpose, and those I 

 use are one-and-a-half inches square. Various cements are used 

 for fixing the rock, the one generally adopted being ordinary 

 hard Canada balsam, although a better one can be made of 

 Venice turpentine and commercial shellac (bleached), which 

 can be procured at any ordinary paint and varnish shop. The 

 proper proportion is about three of Venice turpentine to one 

 of shellac. The turpentine is melted in a water bath, and the 

 shellac is then stirred in, a little at a time, until it is thoroughly 

 incorporated. Test the cement by dropping a little on a cold 

 glass plate, and if it dries practically at once into a hard glassy 

 bead, it is all right. If it is greasy-looking it needs more 

 shellac, and if it is too brittle and chips, more turpentine must be 

 added. When it is of the right consistency, it is poured out 

 on a glass plate, and rolled into sticks like sealing wax, ready 

 for use. 



To cement the rock to the glass, put the latter on a hot 

 plate over a spirit lamp and place a small portion of the 

 cementing material in the centre. At the same time lay the 

 rock on the hot plate to warm. When the cement melts and 

 runs, pick up the rock in a pair of forceps and place it in the 

 centre of the cement. Press down hard to squeeze out the 

 excess of cement, taking care that no bubbles remain between 

 the rock and the glass. If there are any, the rock must be 

 melted off and the work started again. When the rock is 

 cemented satisfactorily the glass plate is put away to get cold, 

 say. for about an hour. Take care when heating the cement not 

 to make it too hot so that it smokes, as this dri\es oft" too 

 much of the turpentine and makes it brittle. 



.■\ method that I have found very satisfactory for fastening 

 the rock to the plate glass holders is to use gum arable, made 

 into a thick solution about the consistency of treacle. .A drop 

 of this is put in the centre of the glass, the rock is placed in 

 position and the excess squeezed out gently with a slight 

 twisting motion. This will dry in about twenty-four hours and 

 the rock can then be ground down in the usual wa\'. 



Having securely fixed the rock to the glass plate, the next 

 step is to grind it down siifficiently thin. If the work is being 

 done entirely bv hand, and the rock has been cemented to the 



