Aug. 17, 1PP3.] 



* KNOWLEDGE ♦ 



103 



fore kept constantly bright and of extreme purity. There 

 being a reservoir of sulphate of copper, the Daniell cell 

 remains constant for weeks. The Bunsen cell lasts Vjut a 

 few hours, the liquid surrounding the carbon, or negative, 

 plate, being rapidly weakened and the plate polarised. In 

 this case, instead of the reduction of a metallic salt by the 

 operation of nascent hydrogen, we get the reduction of a 

 most powerful acid, accompanied by the evolution of highly- 

 irritating fumes. In the Leclanche, the sulphate of copper 

 peculiar to the Daniell, and the nitric acid of the Bunsen, 

 are replaced by a dense black solid, viz., the manganic per- 

 oxide. While, therefore, the Leclancho is, in the strict 

 sense of the term, a single-fluid cell, there are, nevertheless, 

 two compounds employed, one for the absorption of the 

 zinc, the other for the absorption of the hydrogen. 



The continuous action of the cell is, in consequence of 

 the com]iaratively weak affinity between the hydrogen and 

 the manganic peroxide, of extremely short duration. It 

 cannot be relied upon for more than a few minutes at a 

 time, and is, therefore, useless where anything approaching 

 the character of a permanent current is required. It, how- 

 ever, soon recovers itself, or, in other words, becomes de- 

 polarised. 



To make a Leclanche cell (which shall be able to ring a 

 trembling alarm bell through such a length of wire as will 

 reach from the basement to the top story of an ordinary 

 house) the outer cell, which may be of any convenient 

 material, should be of about one quart capacity. It is 

 manifestly not essential that a square glass jar such as 

 we are accustomed to see in the Leclanche cell should be 

 adopted. The zinc may be a piece of half-inch rod — or, 

 better still, a cylinder made from thin sheet zinc. An 

 eighth of an inch would be more than ample thickness, 

 and would help to give a current of increased duration, 

 besides considerably reducing the resistance of the cell. 

 As there is no acid in the cell, and as there is no chemical 

 action going on when the cell is not giving a current, 

 there is no necessity to amalgamate the zinc. The con- 

 nection should be made by soldering a piece of copper wire 

 or strap on to the zinc, and carefully covering the joint 

 and the wire (except the remote end of it) with some 

 pitchy substance to protect them against the ammoniacal 

 vapoui's which are evolved as the solution gets saturated. 

 Crushed carbon may be obtained at 2d. per lb., and man- 

 ganic-peroxide at 3d. The carbon rod will cost a few 

 pence, and will then require to be carefully capped. This 

 is done by first dipping the heated top of the rod in melted 

 paraffin wax, and, after allowing an inch or so of the rod 

 to become saturated, melting on (with the aid of a mould) 

 a lead cap. A brass binding-screw of a compact form 

 should be so placed into the mould as to be fastened into 

 the cap. The cap and exposed parts of the terminal then 

 require to be painted with pitch. These rods, already 

 capped, may, however, be purchased in some places for 

 something less than a shilling. The crystallized sal am- 

 moniac costs Od. per lb. 



Where the Leclanch6 is only required for a few minutes 

 daily it will last for months without any further attention 

 than the addition of a little water. 



The electro-motive force of the cell is 1'5 to I'G volts. 

 The resistance varies, of course, with the size, but it is very 

 low, the one above detailed being about 2 ohms. 



The peculiar traits of the Leclanche cell preclude its 

 adoption for permanent current working, although modifi- 

 cations of it, to Vie hereafter deserilied, are being used v(>ry 

 extensively in the British telegraph service. its great 

 forte is its ability to supply powerful intermittent currents 

 at a comparatively insignificant cost. Of the little atten- 

 tion it requires wu have already spoken. 



As we have now reached the length of our tether, we 

 must defer a few more remarks on this highly interesting 

 cell till another opportunity presents itself. 



PLEASANT HOURS WITH THE 

 MICROSCOPE. 



By Henry .J. Slack, F.C.S., F.R.M.S. 



ONE great advantage of the microscope arises from the 

 facility with which it can illustrate many of the most 

 important principles of scientific investigation. Let us 

 consider, for example, the hairs of plants. First comes the 

 inquiry, What is a plant hair ? The popular answer might 

 be, " Anything hair-like that grows from a plant ;" but this 

 would not suffice, as many appendages of plants must be 

 grouped with hairs, although thpy depart widely from the 

 conception of a hair, founded upon the protective and 

 decorative covering of the human head. Take up any 

 hairy leaf or flower-petal, and with a sharp penknife cut 

 some of the hairs off" just below their base. It is seen that 

 this may be done by removing a piece of the epidermis, 

 without penetrating into the leaf-substance. Go to a rose- 

 tree, with stout, sharp thorns. Press one of these strongly 

 on one side, and it will break off" without injuring the wood 

 of the stem from which it is taken. Next pull a thorn 

 off a small branch of quickset, and some of the 

 wood comes with it. Here, then, is a distinction of im- 

 portance. Hairs and rose-spines are structures growing 

 out of the epidermis, and in any case all objects ranking as 

 hairs spring " from the layer of cells which always remain 

 outermost in roots, stems, and leaves, whether these out- 

 growths occur as simple utricular (little liladder-like) pro- 

 tuberances, rows of cells, plates of cells, or masses of 

 tissue, or have the physiologiral character of woolly 

 envelopes of the young leaves, rootrlike absorbing organs 

 (mosses), glands, prickles, or spore capsules (ferns)." So 

 says Sachs, in his "Text-Book of Botany." Amongst 

 animals we have some similar variations from soft, thin 

 hairs, to sheep wool, bristles of hogs, cats' whisker.s, por- 

 cupines' quills, and rhinoceros' horns. The horn of the 

 rhinoceros, says Owen, " consists of a uniform compact 

 mass of epidermal fibres," that is hairs, and much the same 

 may be said of rose-spines. The thorns of May-trees, &a, 

 belong to the wood, somewhat as the horns of deer 

 " consist wholly of bone " (Owen). 



There are two different scientific ways of studying the 

 parts or organs of plants and animals — one, the morpho- 

 logical, which investigates how they grow, and from what 

 parent formation ; and the other, the physiological, which 

 looks into the functions they perform, or the services they 

 render. It is very often found that parts which are mor- 

 phologically similar, have quite different uses. Thus, hairs 

 of plants, which protect a young bud or leaf surface, are 

 not like rose-spines, nor in their function the same. Plant 

 hairs which are glandular, and secrete a particular fluid, 

 are different in function from others which oppose a me- 

 chanical obstacle to the entrance or exit of certain insects, 

 and thus act as aids to the process of fertilisation. Similar 

 iliu.strations of siuiilarity of origin and great difference 

 of function abound in the animal world, and as a familiar 

 example, we find the morphologist regarding the biting 

 jaws of tlie spider as modified feelers (antenna^), and the 

 nipping claws of scorpions as modified maxillary palpi. 



Plant hairs of all sorts recognised by the morphologist 

 are technically called trichomes, which means hair-sort of 

 things in origin and structure, whatever may be tlieir use 

 or shape. The simplest hairs are unicellular ; other hairs 



