3i6 



SCIENTIFIC NEWS. 



[April 6, iS 



Whatever the change is, we can bring the plates back to 

 a condition in which they are again ready to give a 

 current of electricity, by sending a current in the oppo- 

 site direction until no further chemical change takes 

 place, and the passage of the current merely decomposes 

 the water into oxygen and hydrogen, which come off in 

 bubbles. 



Such a pair of plates, however, would not last for any 

 length of time ; the oxides would be liable to peel off, 

 and the plates would warp and bend from unequal 

 action. In practice, the plates are made of an alloy 

 which resists the acid better than pure lead, and which 

 is nearly as strong as iron. The plates are cast in the 

 form of a grid or honeycomb, and the oxides, as pastes, 

 are pressed into the holes, and thus not only is a much 

 larger quantitj' of active material used, but it is firmly 

 held in the holes of the plates. 



The charging current should be of sufficient strength, 

 or the process will go on very slowly, while it must not 

 proceed too fast, or the plates will buckle, and the plugs 

 of oxide may fall out. Damage may also be done if they 

 be discharged too rapidly, and if they be allowed to 

 " run down " the sulphuric acid will attack them, and 

 form a white insoluble coating which will spoil them. 

 External leaks by which one cell, as a box of plates is 

 called, may run down, and internal leaks or short cir- 

 cuits by which a cell may be rapidly discharged, have to 

 be prevented and detected, and the expansion and con- 

 traction of the plates must be provided for. The 

 strength of the acid should be measured from time to 

 time, and kept to the right density, and precautions must 

 be taken against the scaling of the plates and the loosen- 

 ing of the oxide in the form of mud. These and other 

 causes of failure have done much to retard the more 

 general use of electric accumulators ; but one by one 

 precautions have been taken and improvements have 

 been effected by which a set of accumulators can now 

 be placed under the charge of any intelhgent workman. 

 Considering the state of the few sets of batteries, which 

 have been in use for more than three years, and the 

 improvements which have since been made, these 

 accumulators, if carefully managed, should last for five 

 or six years. 



Strange as it may appear, only one book has been 

 written on this subject, and we gladly draw attention to 

 it, as its value is unquestionable for those who are con- 

 cerned with private electric light installations. We refer 

 to the " Management of Accumulators " (Whittaker and 

 Co.), by Sir David Salomons, who has done so much 

 useful experimental work in many branches of electrical 

 engineering. From an electrical point of view, his 

 residence in Kent is a model, and his workshops and 

 laboratories are also fitted up in the most complete and 

 elaborate way. Our illustration shows the accumulator 

 or battery room at Broomhili, and the convenient method 

 adopted for handling the cells by means of a balanced 

 travelling pulley. The author gives directions for the 

 management of the accumulators, and he describes the 

 various difficulties that may arise, their causes, and the 

 means for their prevention and cure. This little book 

 has just reached its third edition, and although we 

 cannot agree with all that is stated on the subject of the 

 initial cost and the working expenses of the electrical 

 plant said to be necessary for a private house, the 

 general information it contains en the management of 

 accumulators cannot but be useful to all interested in 

 these appliances. 



MICROSCOPIC WORK FOR YOUNG 

 ENTOMOLOGISTS. 



WE propose to give a few simple directions to the 

 beginner who wants to know how to mount the 

 hard parts of common insects. The cockroach is by far 

 the best subject for practice. It is so common that the 

 material may be freely and even wastefully used, without 

 loss to mankind or womankind ; the parts are tough 

 enough to bear rough handling ; and the creature is one 

 of those primitive types which constitute excellent 

 points de rcpere for the student. We shall show how to 

 treat the head of the cockroach, for after this is mastered 

 the rest of the body is easy by comparison. 



Cut off the head, and divide into front and back halves 

 with a sharp scalpel. The stout mandibles should come 

 off with the front half; the other jaws with the hinder 

 half. Several heads should be treated one after another, 

 till the operation can be performed quickly and well. 

 Cut the antennae short with scissors. Put all the success- 

 ful preparations into water, till the time.comes for clearing. ' 

 From a fresh specimen strip off the labium, which 

 hangs down from the back of the head. The labium 

 must be grasped between the thumb and the scalpel, and 

 torn off gently. Remove, also, the maxillae of the first 

 pair with scalpel or fine scissors. When the labium and 

 first pair of maxillae have been stripped off, the rest of the 

 head is useful for demonstration of the tentorium, or inter- 

 nal skeleton, and may be popped into water with the rest.* 

 The next thing is to dissolve away all the soft parts, 

 muscles, nerves, brain, and fat. This is soon done by 

 boiling in a porcelain saucer of liquor potassse. After 

 five minutes' boiling, pour off the solution, wash two or 

 three times with water, then add dilute acetic acid, and 

 leave the parts to soak in it for a few minutes. Wash 

 again with water, and afterwards transfer successively to 

 methylated alcohol, absolute alcohol, and oil of cloves. 

 Ten minutes in each fluid will generallj' be enough. 

 The parts are then ready for mounting in balsam. 

 Arrange each preparation on a cover-glass. The oil of 

 cloves which adheres need not be removed. Place three 

 small glass beads, or other fragments of glass, on the 

 slip, so as to receive the edges of the cover, and keep off 

 pressure from the preparation. This precaution is not 

 necessary in the case of verj^ small objects, such as the 

 labium. Lower the slip, and then add the balsam from 

 one side. Hard balsam, thinned with benzine to the 

 consistence of cream, is best. When the balsam is quite 

 hard, trim the edges, and clean with a rag dipped in 

 benzine, carefully avoiding all dangerous pressure upon 

 the cover. The dissection of the tentorium may be 

 mounted dry in a cell, as follows : — Cement an ebonite 

 ring to a glass sUp with balsam. Line the bottom of the 

 cavity with a circle of black paper, and fix the head in the 

 centre with any good transpa rent cement, such as coaguline. 

 After a few evenings' practice, any of the hard parts of 

 the cockroach can be dealt with. Go on with the mouth 

 parts of other insects. Beetles are very easy, except 

 when of microscopic size. Where the parts are flexible 

 and long, as in Lepidoptera, some management and skill 

 are wanted. Brushes of various sizes and bristles, 

 mounted in handles, are useful in laying out such pre- 

 parations. Patience and mother-wit will carry the 

 operator through all his difficulties. 



* Descriptions of the head of the cockroach will be found in 

 Huxley's " Comparative Anatomy of Invertebrates," or Miall and 

 Denny on " The Cockroach." 



