December 2, 1895.] 



KNOWLEDGE 



271 



pathogenic organisms. Ttiis contamination may be caused 

 by leakage from drains in the neighbourhood of the supply 

 pipes, or by aerial infection of cisterns. It is on these 

 grounds chiefly that at the present time the sterilization of 

 water at the supply works is not advocated, and only the 

 partial bacterial tiltration aimed at. The possibility of 

 pollution from these two causes renders it highly important 

 that some adequate system of domestic filtration should 

 be adopted by every householder. Unfortunately, the 

 majority of household filters are worse than useless, since 

 many of them do not effect the removal of the contami- 

 nating bacteria, and often, by forming a nidus for their 

 growth, contribute to their development and multiplication 

 in the water sought to be purified by their means. If we 

 take, therefore, the primary duty of a domestic filter to be 

 that of delivering sterile water, it follows that no filter 

 should be purchased without a guarantee from the vendor 

 that it is so constructed as to yield permanently sterile 

 water. Most of the manufacturers are now alive to this 

 standard, and few of the older forms of filter, in which 

 charcoal, sponge, or asbestos, or other similar material was 

 employed as the filtering medium, are now in the market. 

 In filters, as in everything else, the survival of the fittest 

 obtains, and the makers of the older forms see their business 

 gradually disappearing as sounder knowledge on these 

 matters becomes more general. That this efticiency is a 

 possible one has been shown by the recent development of 

 the manufacture of what are called candle filters of unglazed 

 porcelain or earthenware. These filters are the outcome 

 of experiments made in 1871 by Tiegel, a pupil of Klebs in 

 Berne, and by Pasteur, who found that plates of plaster of 

 Paris were efficient in sterilizing bacteriological fluids. 

 The first candle-filters were made privately for such e.^- 

 perimental purposes, and were also used by Koch in Berlin, 

 where they are still manufactured by the Sanitilts Porzellan 

 Fabrik at Charlottenburg, and in this country, filters of a 

 similar composition to those used by Koch are made near 

 Manchester. The advantages of filters of this construction 

 were so obvious that Pasteur in Paris was led to study 

 the question and considerably improved on his original 

 idea, which, in the hands of Chamberland, has given rise 

 to the Pasteur-Chamberland pattern, which has been 

 extensively adopted by the French Army, and is now well 

 known in this country. Both the Pasteur filter and those 

 manufactured in Germany are now made of earthenware. 

 Other materials have been used for the production of 

 candle-filters besides clay. Of these modifications, the best 

 known in this country is the Berkefeld-Nordtmeyer filter, 

 which is made of infusorial earth. These candles cannot 

 be sterilized by baking, like those made of clay, as they 

 are more fragile and are liable to fall to pieces. They can 

 however, be readily and conveniently sterilized by placing 

 them in cold water and then gradually heating the water 

 to the boiling point and allowing the candle to remain at 

 this temperature for three or four hours. Dr. Guinochet, 

 in reviewing the evidence on filters of this pattern, con- 

 cludes that the material from which they are made makes 

 it difficult to ensure absolute sterility in the filtrate in all 

 cases, and several investigators have shown that infusorial 

 earth has also the disadvantage of allowing the organisms 

 which are retained by the filter to subsequently grow through 

 the walls of the candle, and thus contaminate the filtered 

 water after the filter has been in operation for some days. 

 This difficulty can be overcome by periodically re-sterilizing 

 the filter, but any such procedure is open to the objection 

 that it is impossible to know, without an actual bacterio- 

 logical examination, whether the filter is yielding sterile or 

 re-contaminated water after it has been at work for some 

 time. Dr. Plagge suggests that for household use two 



Berkefeld filters should be used and sterilized alternately 

 every day. Mons. F. Garros has also suggested the use of 

 asbestos for the construction of candle-filters, and these are 

 now known in France under the name of the " Filtre 

 Mallie." Experiments with these filters have shown that 

 they yield an absolutely sterile filtrate, and that they filter 

 very rapidly. They can be used in conjunction with a 

 preliminary purifier of charcoal or glass wool, which, by 

 removing the suspended particles in the water, prevents 

 the fine pores of the asbestos porcelain from becoming too 

 rapidly choked. At present, however, the Filtre Mallio 

 has only been a short time before the public, and prolonged 

 experiments upon its efficiency are not yet available. It 

 seems, however, to be likely to rank as one of the filters 

 which can be safely used without fear of breaking down 

 after long action, and if the inventor could succeed in 

 rendering the material a little less fragile, it would be one 

 of the best filters at present introduced. I understand, 

 however, that this fragdity has been in part overcome by 

 making the candles of a mixture of kaolin and asbestos, 

 so that the composition of the filtering material will 

 approach more and more closely as the percentage of kaolin 

 is increased to that used in the Pasteur-Chamberland filters. 

 In all these candle-filters, even when they are used under 

 a head of water, the rate of filtration is comparatively 

 slow. On the other hand, if by their use an absolutely 

 sterile filtrate can be ensured, they are to be preferred to 

 all forms of filters which yield a larger supply of water 

 per hour, but which do not profess to remove the whole of 

 the micro-organisms present in the water. This difficulty 

 has been overcome by using several candles instead of one, 

 and thus having what is termed a battery of filters of any 

 desired number. In selecting a filter, then, the principal 

 point to attend to after one is satisfied that initially the 

 filter yields sterile water, is the length of time that such a 

 filter will remain in an efficient condition. This varies 

 with the different kinds of material employed, and although 

 the present evidence seems to show that the kaolin 

 candles, as manufactured by the Pasteur-Chamberland 

 Company, are the best in this respect, they also may allow 

 organisms to grow through them if they are not periodically 

 cleaned. The Berkefeld filters are known to allow this 

 growth through to take place in a very short time, and 

 whenever they are used they should be frequently sterilized. 



All candle-filters are open to the objection that 

 they may be imperfectly baked or cracked subsequently, 

 and thus allow the passage of unfiltered water into 

 the filtrate. A very good teat as to whether such a 

 faulty filter is being used or not is to connect it with 

 an air pump, and note the pressure of air required to 

 produce a current of air bubbles through the filter placed 

 in a jar of water. A Pasteur-Chamberland filter may be 

 considered faulty if it allows air bubbles to pass with a 

 less pressure than eight pounds above the atmospheric 

 pressure, or half an atmosphere. Other materials will not 

 withstand such a pressure, and it is in those cases that 

 one finds the micro-organisms passing through the filtering 

 material. A filtered sterile water is, of course, likely to 

 become again contaminated if it is exposed to the air, and 

 it is therefore desirable, with filters which only yield their 

 water at a very slow rate, that the receiving vessel should 

 be protected fi-om the air. 



In testing a filter for its power of arresting bacteria, it 

 is important to recollect that many natural waters have 

 not the power of sustaining the life of certain specific 

 organisms, and it is therefore essential, in forming an 

 opinion upon the reliability of such evidence, that control 

 experiments should be simultaneously recorded, showing 

 that the particular organisms used in the filtering experi- 



