In regard to filtering through cotton wool, we have known many who had trouble in getting the cotton wet enough to let the solution filter through easily. We use just enough cotton to be held in the neck of the funnel and stay there without being forced through; to wet it, we lay it on the fingers of one hand, patting it with the other hand, while the water is running on it; then transfer it, upside down, to the other hand, and wet and pat the other side. This gets it thoroughly wet, when the water is partly squeezed out of it, and it can be placed in the funnel pressing it partly into the neck, so that the solution will not flow under it when poured in.
The cotton wool does well for all ordinary filtering, but for very nic,e work, filtering paper is used. This usually comes cut in round discs from six to ten inches in diameter. To use it, the paper is folded through the middle and then folded again, when it can be opened like an inverted cone and laid in the funnel. There is another way of folding the paper, (in use by chemists,) bending it out and in, the corrugations largely increasing the filtering surface, the manner of doing which cannot very well be described without numerous illustrations.
In weighing chemicals always lay them on a clean, sheet of paper in the scale pan, and, where a few grains only are to be weighed, balance this paper with an equal weight in the other pan.
In preparing solutions, weigh or measure every chemical carefully. Formulas are not prepared by guess-work; when cer tain numbers of grains or ounces are given, they mean exactly those quantities. Carelessness here is simply the forerunner of failure.
We knew a gentleman who spent a whole summer photograph ing, and gave it up because nearly every negative he made was poor. He had neither graduating glass to measure his fluids, nor scales to weigh his chemicals, mixing everything by guess. If by happy guessing he was able to secure a tolerable negative, he was quite likely to let it spoil through insufficient washing after the hypo. bath.