“591. In using these test papers with a fluid suspected to contain free acid or alkali, or knowing that one of these substances is predominant, to ascertain which is so, all that is necessary is to moisten them with the liquid, and observe the change: if the fluid be acid, the blue colour of the litmus will immediately become red; if alkaline, the yellow colour of the turmeric will be changed to a brown. The moistening may be effected by dipping the paper into the liquid; but a better method is to touch the edge of the slip with a rod dipped in the fluid. In the latter case there is no risk of contamination to the fluid from the paper, and only a very minute quantity of the liquid is used at once.
“592. These trials must be made by day-light; artificial light not permitting that just estimation of the changes by which the presence of a small excess of acid or alkali is to be determined. As the proportion of free acid or alkali diminishes, the intensity of the new tint produced upon the paper is also diminished; and when in very small quantity, it requires considerable attention before a decision can be arrived at. The test paper should occasionally be touched with pure water in the immediate neighbourhood of the part where the solution has been applied, for any change in appearance that may have occurred, not due to mere moistening, is then readily perceived.
“593. Although acid is generally tested for by litmus paper, and alkali by turmeric paper, yet the former is sometimes used advantageously for the latter purpose, being first slightly reddened, either by exposure to the air, or by momentary contact with muriatic acid fumes. When the [p280] paper thus modified is used to detect a free alkali, instead of turmeric paper, that substance is indicated by the restoration of the original blue colour. Litmus paper is best slightly reddened for this use, by putting a drop or two of muriatic acid into a large jar, allowing it to stand a few minutes, and then bringing the paper towards the mouth of the jar, or carefully placing it within: so soon as the blue tint has become slightly reddened, the paper should be removed for use. If too much acid be imparted to the paper, the delicacy of its indications is injured, because of the greater quantity of alkali required to neutralize the acid, and restore the blue colour. For the same reason a paper free from alkali or carbonate of lime has been recommended for the preparation of these tests: for these impurities, combining with a minute portion of acid, neutralize it, and thus prevent that delicacy of indication which the test paper ought and may be made to possess.”
The mode of determining the value of alcaline substances, or “alcalimetry,” is described at length in this section. Our readers, however, will here recollect that there is an error respecting the specific gravity of the acid, which Mr. Faraday has corrected at page 221 of the present volume of this Journal. The thirteenth section is allotted to crucible operations, and the fourteenth to furnace tube operations. They are full of minute and admirable instructions, evidently deduced from long experience, and detailed with the same precision and clearness which we have already eulogised. The fifteenth section, which occupies nearly a hundred pages, relates to “pneumatic manipulation, or management of gases.” Every paragraph of the instructions here given will be found to contain something of importance to the student; it is, indeed, a valuable essay upon a difficult and nice department of chemical research.
Under the head “Tube Chemistry,” in the sixteenth section, a variety of means are pointed out, of working with and employing glass-tubes, as substitutes for more expensive and formal apparatus. Indeed, the young chemist cannot do better than practise the art of bending, drawing out, and sealing tubes, as here directed, (and in the nineteenth section,) by which he will soon gain the requisite dexterity in forming them into test tubes, retorts, and so on, and be enabled to furnish his laboratory with a quantity of very useful vessels and apparatus, at a very moderate expense.
The application of electricity to chemical purposes forms the subject of the seventeenth section, in which the [p281] management of electrical machines and apparatus is described, and the circumstances necessary to facilitate investigation and insure success are pointed out. To this succeed the management and composition of lutes, and a chapter on bending blowing, and cutting glass.
Cleanliness, order, and regularity are of the utmost importance in the laboratory; and though the appearance of the chemist himself is often such that he appears “to doat upon dirt,” the strictest nicety must generally be observed in the state of his utensils and apparatus. These matters must, indeed, generally engage his personal attention; and it is not sufficient that glasses and other vessels be merely washed and wiped in the usual way, but they are generally required to be free from the minutest portions of adhering matter. A section is accordingly appropriated to the subject of cleanliness and cleansing, in which, and in that which follows it, entitled “General Rules for young Experimenters,” much information is conveyed that will prove useful to those who are commencing the practice of experimental inquiries in chemistry, and also to such as, having made some progress, have indulged themselves in slovenly habits. Macquer’s observations on this subject, as quoted by our author, are so much to the purpose, and so well deserving the serious attention of the young chemist, that we shall stand excused for inserting them in this place. He says, “A persuasion must exist that arrangement, order, and cleanliness, are essentially necessary in a chemical laboratory. Every vessel and utensil ought to be well cleansed as often it is used, and put again into its place; labels ought to be attached to all the substances, mixtures, and products of operations which are preserved in bottles or otherwise; these should be examined and cleansed from time to time, and the labels renewed when required. These cares, although they seem to be trifling, are, notwithstanding, the most fatiguing and tedious, but the most important, and often the least observed. When a person is keenly engaged, experiments succeed each other quickly; some seem nearly to decide the matter, and others suggest new ideas; he cannot but proceed to them immediately, and he is led from one to another; he thinks he shall easily know again the products of his first experiments, and therefore he does not take time to put them in order; he prosecutes with eagerness the experiments which he has last thought of, and in the mean time the vessels employed, the glasses and bottles filled, so accumulate that he cannot any longer distinguish them; or at least he is [p282] uncertain concerning many of his former products. This evil is increased, if a new series of operations succeed, and occupy all the laboratory; or if he be obliged to quit the place for some time, every thing then goes into confusion. Hence it frequently happens that he loses the fruits of much labour, and that he must throw away almost all the products of his experiments.
“The only method of avoiding these inconveniences is to employ the cares and attentions, above mentioned. It is indeed unpleasant and very difficult continually to stop in the midst of the most interesting researches, and to employ much valuable time in cleaning and arranging vessels and attaching labels. These employments are capable of cooling and retarding the progress of genius, and are tedious and disgusting; but they are nevertheless necessary. Those persons whose fortunes enable them to have an assistant operator, on whose accuracy and intelligence they can depend, avoid many of these disagreeable circumstances; but they ought nevertheless to attend to the execution of these things. We cannot depend too much on ourselves in these matters, however minute, on account of their consequences. This becomes even indispensable when the experiments are to be kept secret, at least for a time, which is very common and often necessary in chemistry.
“When new researches and inquiries are made, the mixtures, results, and products of all the operations ought to be kept a long time well ticketed and noted. It frequently happens that at the end of some time these things present very singular phenomena, which would never have been suspected. There are many beautiful discoveries in chemistry which were made in this manner, and certainly a much greater number which have been lost, because the products have been thrown away too hastily, or because they could not be recognised after the changes which happened to them.”
The uses of equivalents, and the method of employing Dr. Wollaston’s scale, form the subject of the twenty-second section of Mr. Faraday’s book; and of the concluding sections, the twenty-third contains a quantity of miscellaneous remarks, and the twenty-fourth is appropriated to “a course of inductive and instructive practices;” that is, to a selection of minute instructions respecting the use of instruments, and the performance of operations.