M 



i Mi: MIST HV. 



[grANTITATIVB ANALYSIS. 



capable of weighing M low as grains, may be em- 

 ployed ; but in estimating the quantities of constituent*, 



twlwicv which will weigh accurately to the -nfoijth part of 

 a grain, U absolutely needed. Of courne, we cannot 

 mootnmend the initrument made by any particular manu- 

 facturer ; bat we may point out various particular*, to 



hich the attention of our readen miut be directed, if 

 they wo ild obtain reliable result* in their investigations. 



In the balance, we obeerve a lever whose fulcrum i 

 midway between iU two extremities, and whicli, when 

 perfect, should lie in an exact horizontal line, or parall. ! to 

 one drawn through tho fulcrum. The operation of writh- 

 ing consist* in the addition of two weights, one at eitlior 

 extremity of tin- Icier ; which, if in excess of each other, 

 hall cause the lever to be depressed beucath the hori- 

 zontal, on the side on which the excess rests ; but which, 

 if equal to each other, shall allow the whole lever to be 

 parallel to the horizontal line of rest Now, at first 

 sight it would seem to be an easy matter, according to 

 the skill of the workman, to construct an accurate 

 balance ; but one or two points have to be attended to. 

 That of the centre of gravity U an essential one ; for if 

 that be in the centre of the lever, then the scales will 

 have a tendency to "set" when over-loaded that is, 

 the horizontal line will not be regained when even 

 exactly the same weights are on each part of the scale. 

 This difficulty is overcome by so arranging the centre of 

 oscillation, or that on which the beam turns, with the 

 centre of gravity, so that the latter may be above the 

 former.: In proportion as this point is attended to, 

 then, within certain limits, does the value of the 

 balance depend. The homogeneous character of the 

 metal of which the balance is composed, is another mat- 

 ter of some importance ; for although, theoretically, we 

 pnrume the lever to be inflexible, it is really not so ; 

 and an excess of weight, or carelessness of weighing, may 

 seriously damage the balance-beam in many cases partly 

 through a want of homogeneity in its parts. 



In the operation of weighing, many precautions have 

 to be attended to. As we have already said, no sub- 

 stance should be weighed except on paper or in a glass 

 tube ; for if the operator be neglectful of this, the scale- 

 pan may become oxidated, and so prove heavier than its 

 fellow. Besides, portions of the substance are sure to 

 adhere ; and the rough usage required to remove them is 

 totally opposed to the delicate treatment a good balance 

 should receive at the hands of its owner. Heated substances 

 should not be weighed, because, as they would tend to 

 expand the arm of the beam under which they are placed, 

 ther would seem to weigh heavier than they really did; 

 and hence a serious source of error would be introduced. 



The pans should never be loaded whilst suspended from 

 the arms of the beam. This never need be done ; for in 

 all chemical balances, arrangements are made by which 

 the pans, arms, kc. , can all be suspended simultaneously 

 for the operation of weighing, and be brought to rest 

 again when it is concluded. By such ineanx all undue 

 or unnecessary strain on the arms of the balance is 

 avoided, and a highly possible source of injury removed. 

 The balance should always be kept away from the fumes 

 of acid*, &c. They are generally enclosed in glass cases, 

 which, except when in use, prevent any great risk of 

 injury. 



The analyst will require a considerable variety of 

 weight*, from the fraction of a grain upward ; the smaller 

 being made of platina. Small weight* are readily made 

 from tin- platiria wire by weighing a portion, and then 

 dividing it for halves, quarters, <!<:., of the normal 

 m%Mfc The weight* should not be touched by the 

 finger, but put into, and removed from, the scale-pan by 

 means of a pair of tongs. Small pieces of tin or lead- 

 r small shot, are useful in counterpoising crucibles, 

 *o. They must be kept scrupulously clean. Glass 

 beads are preferable, because they do not acquire an 

 increase of weight by oxidation. 



It must be kept in mind, that many substance*, whilst 



being weighed, acquire a certain amount of moisture 



the air, and so increase in weight; as, for instance, 



causUo pot*.*, soda, baryte*, lime, Ac. The*e are 



fig. 74. 



termed hygmmetric substances ; and such should always 

 be weighed in very thin test-tubes. This is a point of 

 considerable imjwrtance in organic analysis, in which 

 the substances operated on are generally minute in quan- 

 tity, and the black oxide with which they are burned 

 being hygrometric. 



In very delicate balances, which will turn with at least 

 the nfro tn I >art f the weight in the scale say with a 

 load of 500 to 800 grains the indications of an index 

 will assist in a further attainment of accurate weighing, 

 and thereby another decimal figure may be added to the 

 result This, however, can only bo done by an expe- 

 rienced person. Meanwhile, we direct the attention of 

 the tyro to the fact, so that he may gradually acquire a 

 knowledge of the peculiarities of the chemical balance. 



IVforo proceeding to make remarks specially appli- 

 cable to certain precipitates, we may give a few more 

 general directions. The precipitate having been col- 

 lected on the filter, must be carefully washed by means of 

 a gentle stream of water, if insoluble in that liquid ; and 

 this is best done by means of a washing-bottle. This 

 may be of any form or shape : a convenient one ia repre- 

 sented in the margin. It 

 consists of an ordinary 

 glass bottle, fitted wth two 

 glass tubes, having a bore 

 equal to about the tenth 

 of an inch. That marked 

 a, is one from which the 

 water flows ; whilst the 

 air passes through 6, to 

 take the place of the es- 

 caping fluid. It is easily 

 filled by drawing air out 

 l>y the mouth through the 

 tube b, whilst a is im- 

 mersed in a vessel of dis- 

 tilled water. When thus 

 filled, the bottle is vised 

 for washing, as shown in the cut The tube a should 

 be drawn to a jet-like point, so that the stream of water 

 may flow gently on to the precipitate, and not drive 

 holes in the filter. 



The removal of the filter from the funnel requires 

 care, because the filtering-paper has little tenacity, and 

 hence may readily break in pieces. The best plan is to 

 tilt the funnel on one side, and blow between the paper 

 and the glass surface. This will separate them, ami a 

 good hold may then be taken of the filter for its re- 

 moval. The next step is that of drying the precipitate ; 

 and this requires care. In most cases a water-bath is 

 all that is requisite ; and one of these is readily con- 

 structed by means of two evaporating dishes. The filter 

 being placed in one, it is floated in another containing 

 water kept at a boiling state. This or any other similar 

 plan may be adopted. Water-baths, however, made in 

 IJ'Tlin porcelain, may be procured of the iustrument- 

 makers. If a higher temperature be required, as for the 

 evaporation of acids, <fec., then a sand-bath .must be 

 used ; for which purpose an iron or copper bowl, filled 

 with sand, and placed over the spirit-lamp or furnace, 

 will answer. The evaporating dish containing the filter, 

 &o., to be dried, is placed in the sand. In some cases it 

 is desirable to burn the filter, and to weigh the residual 

 precipitate. This is best done in a Berlin or platina 

 crucible. We must here warn our readers never to burn 

 precipitates containing oxides of metals proper, either 

 with paper or in a plat ma crucible. The charcoal of the 

 paper would most likely reduce the metal. And metals 

 such as lead, dec., if coming in contact with a red-hot 

 platina crucible, would render it useless by forming an 

 alloy. The alkalies, earths, and their compounds .- 

 should find a place in a platina vessel at a high tempe- 

 rature. Some substances, during drying, decrepitate ; 

 that is, they cast oil' small portions, owing to the forma- 

 tion of steam, which, expanding between their particles, 

 affords small explosions. Such occasion loss ; and a 

 cover of any kind, such as another evaporating dish, 

 should be inverted over that containing the drying 



