HARDWICKE'S SCIEN CE-G OSSIP. 



S7 



piece of thin glass, is placed upon the brass plate 

 with the centre of the covering glass immediately 

 under the ivory knob. The knob is then screwed 

 gently down sufficiently firmly to hold the covering- 

 glass steady. The slide is next warmed over a 

 spirit-lamp, there being an opening in the brass 

 plate to admit of this being more readily done, and 

 a drop of Canada balsam being placed at the edge 

 of the glass cover, is drawn in under it by capillary 

 attraction, entirely surrounding the objects without 

 disarranging them, and completely excluding air- 

 bubbles. If it be desired to avoid the application 

 of heat, a solution of Canada balsam in chloroform 

 or benzole may be applied in the same way without 

 the use of the spirit-lamp. It would doubtless be 

 possible to mount diatoms, &c., arranged in patterns, 

 without the aid of Mr. Smith's little instrument, by 

 proceeding on the same principle, but employing 

 only one of the ordinary wire clips. The disadvan- 

 tages of this would be that the pressure could not be 

 graduated with sulBcieut nicety, and the hold upon 

 the slide and covering glass would be of so unstable 

 a character as to be displaced by any sudden move- 

 ment ; thus destroying at once the symmetry of the 

 objects and the patience of the operator. I am very 

 much afraid that the pressure of the cover without 

 that of the mounting instrument would smash any 

 but the stoutest diatoms : fancy pressing Cosciko- 

 discus coiicmnus or the Pleurosigmas. — R. C. M. 



A PoLAKiscoPE.— Will any of your readers 

 kindly inform me where I can find a figure and 

 description of Mitscherlich's polariscope ? The 

 instrument is referred to in Drs. Dupre and Thudi- 

 chum's treatise on wine, as useful in estimating 

 the amount of sugar in wines. This instrument 

 effects this by ascertaining the amount of rotation 

 imparted to a ray of polarized light in passing 

 through the sample of wine under examination. 

 Perhaps some one can kindly give me a hint how to 

 fit up, cheaply and simply, a polariscope for estimat- 

 ing the quantities of sugars in various solutions. — 

 F. M. S. 



Soft Baxsam.— My old friend and teacher, the 

 late Dr. Arnott, of Glasgow, always strenuously 

 objected to the use of soft balsam, or rather to 

 putting on the cover before it had been hardened. I 

 have now before me one of his letters, written in 

 1858, where he says, "Pray harden the balsam 

 before putting on the cover; the pressure upon 

 your marked slide has either slipped the cover or 

 the object; bear in mind balsam never hardens 

 beneath the cover, and if slides mounted in that 

 way are sent to a hot chmate they would be use- 

 less." I have just removed the cover from a slide 

 mounted twenty years ago, and the balsam is now 

 soft enough to adhere to the fingers. I do not 

 quite understand Mr. Walmsley's directions; for 



instance, he says, "Make a saturated solution of 

 balsam in pure benzole until it is of the consistency 

 of rich cream." Now as balsam is soluble in ben- 

 zole in all proportions, a solution of the consist- 

 ency of cream cannot be said to be a saturated 

 solution. In another place he says, "a drop of 

 balsam is to be placed upon it, followed by the usual 

 core; " and a few lines further on he again alludes 

 to a core, whatever this may be.* Although oil of 

 cloves possesses some advantages over turpentine, it 

 evaporates so slowly that the balsam surrounding 

 the objects on the slides sent to the editor is as soft 

 now as when the cover was put upon it. A further 

 proof of the softness of the balsam beneath the cover 

 is the tendency of coloured cements to run in, and 

 which Mr. Walmsley avoids by using the balsam as 

 a finish. My hard balsam slides have either a ring of 

 asphalte varnish or a mixture of dammar varnish, 

 gold size, and vermilion, and no running in has 

 taken place, although they are placed in racked 

 drawers. Another and important difference exists 

 between soft and hard balsam : their refractive 

 powers are not alike. If a valve of Pleurosigma 

 Balticum, for example, is mounted in soft balsam, it 

 is scarcely visible, and no trace of striation can be 

 detected ; but if the balsam is hardened, they are 

 easily resolved. If Mr. W. has had the number of 

 slides sent to him for the purpose of naming that I 

 have had, and seen the damage done to them by the 

 pressure of one slide upon another, squeezing out 

 the object with the soft balsam, he would have 

 the same horror of it that I have.— -P. Kitton. 



BOTANY. 



IsOETES. — Is there not some little confusion in 

 the account of Isoetes in the March number of 

 Science-Gossip ? According to the latest authori- 

 ties, the British species are, — 1. /. lacustris, L. ; 

 2. /. echinospora, Durieu. This is the rarer form, 

 and according to Hooker ("Student's Flora"), is 

 only a sub-species of the ijreceding. These are the 

 aquatic species. 3. The terrestrial: 1. hystrix, 

 Durieu (not histrix), of which /. Burieucei, Hooker, 

 is a synonym, occurs in Guernsey, at L'Ancresse 

 Bay, Its so-called "spines" are the old leaf-bases, 

 which are " spinescent, dark, horny, and consist 

 of lateral subulate processes, and an intermediate 

 tooth." If by "seeds" the macrospores of Isoetes 

 are intended, it is perfectly intelligible, that, in the 

 absence of the microspores, no result could have 

 followed from their being sown. — B. A. Pryor. 



British Okchids. — Scarcely a number of 

 Science-Gossip appears without containing some 

 reference to our native Orchids. This fact testifies 



* Query "Cover." — (Ed. Scjence-Gossip.) 



