the tenth part of nitrogenous matter which was not removed there was that amount of unfiltered water passing through into the storage receptacle. Now, if the tenth which passed through contained any of the germs of disease, or communicable matter of disease, whether germs or not, that would be equally as fatal to the drinkers as if it were entirely unfiltered water. He had been led to this train of reasoning more particularly because during the last six or eight months he had been testing filters which removed nine-tenths of the decomposible organic matter; and the other tenth would have been removed had the filters been mechanically perfect. He had been making experiments on a very fine precipitate, sulphate of barium, and found that that freely passed through many of the filters, and therefore it became obvious that matters which were very much smaller than that would equally pass through. He then went on with salts of uranium, a still finer precipitate, and found they almost invariably went through. Then it occurred to him it would be well to try if he could not stop this. Both these salts were white, and, therefore, were perfectly visible to the naked eye, or, in the finer particles, under the microscope. He tried to stop it by using finely powdered carbon, animal, vegetable, and mineral. The result was that he found that with carbon of all three kinds, when reduced to much finer particles, they also came through the greater part of the filters he tested

This was most important, because anyone could go home and make similar experiments for himself. In working this out, he found after having used a considerable amount of very fine carbon, he had stopped very nearly the whole of that one-tenth of decomposible matter, and in fact he had decomposed the greater part of that tenth. Knowing the oxydising influence of spongy platinum, he went one step further, and used platinum black. Of course he would not recommend this as being suitable for ordinary filtering purposes, because of its expense, and there might be other objections also; but it did absolutely oxydise and remove every trace of decomposing nitrogenous matter.

That being the case, it was certain that perfect filtration could be obtained. Following this up, he had tried carbon in the finest divided state, so fine that he found by the microscope the particles were finer than the markings on a diatom, for under a 50th power he could not measure them, although he could measure the markings of diatoms. Carbon, therefore, could be obtained, if necessary, in so fine a form of sub-division that an immense amount of oxydising power was thereby obtained. It might be said, how were you going to filter water through that extremely finely divided carbon. He thought possibly that cotton wool, which obstructed all germinal matter passing through, as was shown in the well-known experiments of Professor Tyndall and M. Pasteur, was a means to the end, but soon afterwards he found that cotton wool decomposed in water, and therefore that upset the experiment. He therefore tried two other media, one a peculiar kind of slag wool, and the other asbestos fibre, both of those retained the finest particles of charcoal, and formed a perfect filtering medium as far as developing the oxydising property of finely divided carbon was concerned. Having obtained this result, he must add that he was not aware of any filter now before the public which had no decomposing matter in its construction. The introduction of the cork caused decomposing matter to be mixed with the water, and under the microscope large quantities of moving organisms were to be found in the decomposing matter of the cork itself. The same thing, in a lesser degree, occurred with indiarubber, and, therefore, he laid it down as a fundamental axiom in filter-making that no decomposing matter of any kind should enter into its construction. Further than that, the filtering media should neither pass the finest possible charcoal, nor should they be choked up by it. These two points formed the most important tests he had to bring forward. If they fell back on the old plan of testing filters which we had been in the habit of using for twenty years, they would be misled as to the mechanical deficiencies of which he had spoken of, for it was no use whether one

adopted Dr. Frankland's process for the analysis of water, or the estimation of ammonia, to suppose that because you were able to oxydise three-quarters, or even nine-tenths of the decomposible nitrogenous matter, that the filter itself was therefore of any value. If this were recognised, and people would make the experiment for themselves, he believed better filters would be produced, as purchasers would be able to test the filters by these means. There would be at last a possibility of arresting much of that which was injurious to health; and it was not impossible that they might prevent even the smallest germs, which must be of some size, passing through the very fine medium he had suggested.

Mr. THOMAS SPENCER said they had heard a great deal of the necessity of softening water, and a great many processes for effecting that purpose had been mentioned, but they had heard nothing as to the physiological difference to the animal system of very hard water and very soft water. For forty years he had been going through most of the towns in England, and he could never get that question answered-what was the physiological effect on the system of hard water, as hard as that supplied by the Thames, as compared with soft. He might say that he knew most of the water supplies of the world, and he did not know of a better supply anywhere than that which came from the Thames when properly filtered; he did not mean by any chemical means, but when well filtered mechanically. He had constructed waterworks at Calcutta, and knew many of those on the Continent, and if the Thames water were well filtered, it would be one of the best water supplies to any large town in the kingdom. When he put that question he never got a reply, but was told he did not require any explanation—that salts of lime and magnesia were not desirable. Two years ago, he suggested to the authorities of a town with which he was connected, that to settle this question it would be well to get accounts from the authorities of the different towns, and ascertain where the death-rate was highest-whether

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in towns having a soft water supply, or hard water. That report had now been returned, and no doubt would be in print before long, when it would be found that the deathrate was larger where soft water was used, and this to a much greater extent than he had anticipated. He must say also that he began as a soft water fanatic, but there was no doubt that one reason why soft water was so much recommended was that in many of the towns in the north, calico-printing, and similar works were very numerous, and as soft water was much better for such operations, people naturally supposed that similar water was better for domestic purposes.

Mr. MAIGNEN said he had hoped some paper would have been read on the subject of filtration as a whole. They heard a good deal of how water was contaminated in a very astonishing way, but how to get over the difficulty they had heard very little. He could not attempt to do justice to the matter in the very few minutes he had at his disposal; but what he had to draw attention to was this, that on Thursday last he was in the laboratory of M. Pasteur in Paris, who asked him " how will your filter last?" Now this was a subject worthy a whole day's discussion. The first necessity of filtration was that, when impurities had been collected in the filtering material, it could be easily removed, and thrown away and replaced. Dr. Bartlett had knocked the nail on the head by drawing attention to a very important fact. If a microbe had a body, it was just possible to conceive an impediment, or a series of impedimenta, fine enough to prevent it passing through. Dr. Bartlett told them that he had used charcoal the particles of which were smaller than the smallest known diatom, and if these particles did not come through a particular form of filter, it was conclusive that the diatom itself could not pass through. He echoed the statement made by one of the speakers, that if the London Thames water were properly filtered, it would be the best in the world, and he had reason to hope that this would soon be attained by means of the apparatus shown on the wall.

Dr. HAUGHTON said a great deal had been said in favour of the water they were obliged to drink in London, there being no option as to the source from which it was obtained; and he wished he could heartily endorse all that had been said in its favour, because it was unpleasant to have a bad opinion of that which one had to deal with continually. He listened to Dr. Odling's paper with great admiration until he came to the last sentence; but they were not told there whether his estimate of the London water as compared with the supplies of other towns was founded on its present condition, or on some former period, or on a fair average. He had in his hand an extract referring to the condition of things at the time of the epidemic of 1830; but it was written twenty years later, and those periods were not referred to with reference to the microscopical analyses and examination of the water. It stated that at this time there was hardly a water company in London the water of which did not reveal to the microscope solid particles of sewage floating about in the company of small weeds and sponges, and fine fat pulpy animalcules. At an interval of twenty years there was the same condition; and it was obvious that when such things had happened they might happen again, especially when eels and fish were found in reservoirs where they were not bred. It was not at all infrequent in disputed cases in the Law Courts for chemists to give evidence with regard to the supposed condition of streams; when one would state that the water was dirty, and another would say it was beautiful, and both had made analyses, and both were men of science. With regard to the basis on which this statement with regard to London water was founded, it appeared, so far as he could make out, that the ground upon which Dr. Odling claimed this wonderful excellence was the very ground which, in the carlier part of the paper, he said it was totally inapplicable to put before the general public. For if he only got the chemist's estimate of the amount of organic matter, and that was put forth to the general public as a test for the