HARDWICKE'S SCIENCE-GOSSIP. 



29 



may remain in a saturated solution for three or four 

 hours. To kill animals in an expanded condition it 

 may be used hot, and immediately replaced by cold 

 solution. As an instance of its use, suppose one is 

 dealing with the contents of a surface net, all animals 

 of the most extreme delicacy. To the sea water in 

 which these are swimming about add a small quantity 

 of solution of ferric chloride, which will kill the 

 organisms most effectually. They will now sink to 

 the bottom and form a sediment, from which the sea 

 water may be poured off. Fill up with saturated 

 solution of corrosive sublimate,* and leave for three or 

 four hours. Then thoroughly wash with distilled 

 water, by decantation, until all traces of sublimate 

 have disappeared, and pass up into 70 per cent, 

 spirit in the way mentioned above. Specimens may 

 be now labelled and laid aside, for they will remain 

 unaltered for an apparently indefinite time. 



Now a few words as to staining. In the Cambridge 

 Morphological Laboratory, all the preparations are 

 stained before cutting, and the beautiful results ob- 

 tained there quite dispose of the theory that staining 

 in mass does not give definiteness of outline. There 

 the block of tissue, or the animal, as the case may be, 

 is left in an alum and calcic chloride solution of hema- 

 toxylin for twenty-four hours. It is then washed as 

 rapidly as possible in a solution of '25 per cent, nitric 

 acid in 70 per cent, spirit. This dissolves out 

 superfluous stain, removing it in part from those 

 portions of the tissue which have no great attraction 

 for it, and thus gives differential staining and great 

 definiteness of outline. The time for leaving in the 

 hematoxylin is not of vast importance ; all that is 

 necessary is that the block should have time to 

 become thoroughly filled with the stain. 



Definiteness of outline appears to be much more 

 dependent on perfect dehydration than on staining. 

 The nitric acid solution may be replaced by 70 per 

 cent, spirit, this by 90, and this by absolute. 



By far the most delicate method of imbedding is 

 the following. The stained specimen is left in 

 the absolute alcohol until it is thoroughly permeated 

 (a matter of a few minutes only) ; then, with a pipette, 

 place a quantity of chloroform sufficient to cover the 

 specimen, at the bottom of the small bottle in which 

 it and the absolute alcohol already are, where it will 

 form a lower stratum. The specimen will float in 

 the upper layer of alcohol, and will only gradually 

 sink into the chloroform, the alcohol with which the 

 tissue is soaked being very slowly replaced. When 

 this is accomplished, the upper stratum of alcohol 

 and the chloroform may be drawn off with the pipette, 

 and a little fresh chloroform added. Now shred 

 some paraffin into the bottle, cork loosely, and leave 

 for a short time at a temperature of about 30 C. 

 The paraffin will be slowly dissolved by the chloro- 



* It is found best to make the solution with sea-water when 

 jt is to be used for marine organisms. 



form, the density of the fluid equally slowly changing, 

 and permeate the tissue. Raise the 'temperature to 

 6o° C, and the chloroform will be driven off, and 

 when this is completed, as determined by smell, 

 transfer to a larger quantity of melted paraffin, and 

 imbed in the usual way. 



I am inclined to endorse Mr. Underwood's dictum, 

 that the hardness of the paraffin should be varied 

 according to the surrounding temperature. The 

 temperature of the Cambridge laboratory is tolerably 

 constant, from the method of heating, so I cannot 

 speak from experience. There we use two kinds of 

 paraffin, " hard " and "soft," the latter being of such 

 a low melting-point that it can be moulded in the 

 fingers at the ordinary temperature. The soft paraffin 

 is used first, before passing into hard paraffin. It is 

 also extremely important as an aid to obtaining 

 " ribbons," serving, from its pliable nature, as a 

 cement to unite the different sections together. To 

 this end the little cubes are coated on only two sides 

 with this soft paraffin, in the line in which the ribbons 

 are intended to run. They are generally dipped 

 bodily into some hard melted at a low temperature, 

 and then the soft paraffin is carefully removed with a 

 knife, except where wanted. I find it easiest myself 

 to melt a little on the blade of an old scalpel, 

 and apply as thin a layer as possible in that way, 

 and on the two sides of the cube only on which it is 

 needed. 



None of these statements are intended to traverse 

 Mr. Underbill's remarks, since he was dealing only 

 with insects, and I have had no experience of them. 



W. B. H. 



Cambridge, 



CHAPTERS ON COLOUR. 



By S. A. Notcutt, jun., B.A., B.Sc. 



No. II. 



T 



URNING to the mixture of pigments, we find 

 results entirely different from those arrived at 



Red. 



Or. Yel. 



Gr?. 



Bl. 



Viol. 



Fig. 14.— Diagram of the Three Primary Colour Sensations 

 (1, red ; 2, green ; 3, blue or violet) showing the extent to 

 which they are affected by rays belonging to different parts 

 of the spectrum. (Helmholtz.) 



ty mixing coloured lights. We know that blue and 



