Dee., 1914
T H E J O U R , V S L O F I N D U S T R I A L A N D E X G I N E E RI N G C H E M I S T R Y
Figs. 3 and 4. The instrument is essentially a gas-balance, in which the specific gravity of the gas .under test is compared with t h a t of the surrounding air. This is accomplished differentially, the weight of a column of gas and the weight of an equal column of air being arranged t o act on opposite sides of what is virtually a scale-pan, and the difference in weight registered. The bell of the gravitometer may be considered as a scalepan on which the pressure below is due t o the pressure of the atmosphere on the lower surface, and the pressure above t o the pressure of a column of gas added t o the pressure of the atmosphere a t the top of t h a t column, the aggregate being less, in the case of coal-gas, than t h a t of the atmospheric pressure below the bell. Gas is always passing up the tube to supply the burner a t the top, and under these conditions is under absolutely the same conditions of temperature and pressure as that of the surrounding air. It will thus be seen t h a t the pointer of the gravitometer indicates the weight of t h a t column of gas of a height equal t o t h a t of the tube. For example, with a bell of 6 in. diameter and a height of tube of 30 in., the gas of, say, 0.9 specific eravitv is 1.5.9 grams. The pressure or weight on the top of the bell is in direct proportion t o the specific gravity of the gas in the tube, and is, in addition, proportional t o the height of the tube. Thus, if the tube were 60 in. in height instead of 30 in., the pressure on the bell, due to the weight of the gas, would be 30.38 grams, and the movement of the pointer across the scale would be twice t h a t in the case of the 30-in. tube. Mechanical considerations constitute the only reason for not using a longer scale; indeed, by employing tubes of sufficient height, it is possible to measure specific gravity to any required degree of accuracy. Referring t o Fig. 3, which shows diagrammatically the action of the gravitometer, A reprerents a light aluminium bell, sealed in oil contained in the annular tank B. The Bell is covered b y a shell, C, which is not in contact with it. The top of the shell carries a tube, D. T h e bell is free t o move up and down, and is carried by a vertical support connected to the end of the balance-beam G by a hanging chain. The other end of the beam carries a hanging weight, H. The chains connected to both H and A pass over circular arcs. A pointer, K, and a gravitycontrol weight, L, complete the essentials of the instrument. On a stream of gas passing slowly over the bell A and u p the tube D, the pressure on the top of the bell is diminished and the pointer K swings into a new position of equilibrium. The scale over which the pointer moves is graduated t o read or t o record specific gravity directly. T H E IMPERMEABILITY O F CONCRETE
In a paper recently read before the Western Society of Engineers and quoted in Engineering (London), 98 (1914), 483, Professor M. 0. Withey described the results of a series of tests on the permeability of concrete, which have been made a t the University of Wisconsin. T h e materials used were Portland cement, sand ranging in weight from 104.5 lbs. to 1 1 2 . 2 lbs. per cu. ft., and gravel weighing from 107.3 lbs. to 190.3 lbs. per cu. ft. Eighty-eight of the test-pieces were made with a I : 1 ~ :~ 3 2 mixture by volume, and sixty-seven with a I : 2 : 4 mixture, and there were ninety-eight specimens proportioned with I part by weight of concrete t o 9 parts by weight of aggregate. None of the concretes proved absolutely water-tight in the sense t h a t they would not absorb water, but most were so impervious t h a t there was no visible evidence of flow. The signs of dampness on the bottom of the 'specimens increased with increasing humidity of t h e atmosphere. With mixture of I part of cement t o 7 parts of aggregate the average rate of flow during a period of fifty hours was under 0 . 0 0 1 gal. per sq. f t . per hr., when the pressure was 40 lbs. per sq. in. With the I t o 9 mixtures, prac-
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tically water-tight concrete could, it was found, be obtained by suitably grading the sand and gravel. Richer mixtures, such as the I : I l,'2 : 3, proved very impervious, but Professor Withey remarks t h a t such rich mixtures show considerable volume changes when alternately wetted and dried. To secure impermeability great care is needed in mixing the concrete, especially when the proportion of cement is small. If mixed too dry, the concrete cannot be properly compacted. The best results were obtained by mixing the materials dry for l / ' d t o ', min., and then continuing the process after adding the water for I ~ ! to ' ~ 2 min. with I to 9 concrete, or for I min. with the rich I : 11, : 3 mixture. Proper curing of the concrete greatly adds t o its impermeability. Premature drying destroys the imperviousness of the lean mixture, seriously impairs t h a t of the I : z : 4 mixture, and appreciably diminishes the watertightness of the rich mixture. Thus, thin sections of 6 in. t o 8 in. in thickness should be, he concludes, kept damp for one month for lean mixtures, or two weeks for a rich one. Engineering also quotes (rol. cit., 446) from Science Conspectus, a publication of t h e Society of Arts of the Massachusetts Institute of Technology, some particulars of a n interesting series of experiments now being carried out by the Aberthaw Construction Company in order to disprove the theory t h a t the combined effects of sea-water and frost rapidly destroy concrete structures. With this object in view, twenty-four concrete columns, 16 f t . long, 16 in. square, and reinforced with bars near t h e corners, were constructed in January, 1909, and immersed in the water a t Boston Navy Yard. They were suspended in such a manner t h a t a t high tide the water reaches nearly t o the top of the column, and falls a t low tide nearly to the bottom. In cold weather the columns are thus alternately thawed and frozen as the tide rises and falls. The columns were made with various qualities of concrete mixed dry, plastic, and very wet. Different qualities of cement were used, and the effects of waterproofing materials, clay, and other additions t o the concrete are being studied.' One of the columns was mixed with salt water, b u t this was unfortunately lost in handling. KO final conclusions are, of course, possible yet; many years must, in fact, elapse before it will be possible to say which kind of concrete is most permanent. When examined in December last many of the specimens were practically unaffected, b u t others were badly eroded. ,4s might be expected, the best ,results were given by the specimens richest in cement and mixed wet. For instance, of two columns made with I part of cement t o I of sand and z of stone, the one mixed dry was badly eroded over the whole of its length; whereas the other, which was mixed very wet, was only slightly pitted. Again, of two.specimens made with slag cement in the ratios of I : I : 2 and I : 3 : 6, respectively, and both mixed wet, the former was in excellent condition, with only very slight pitting, while in the latter all the corners had gone and the reinforcement was exposed in places. The part of this specimen which was continuously immersed was, however, in very fair condition. T h e experiments are being continued, and doubtless some very interesting results will be obtained in time. PUTTING ELECTROTYPING INDUSTRY ON MORE SCIENTIFIC BASIS -4t a meeting in New P o r k on October 7 , 1914, the International Association of Electrotypers appointed a committee to cooperate with the Bureau of Standards in a study of the conditions in the electrotyping industry, with a view to assisting in placing it upon a more scientific basis. A preliminary circular giving simple directions for testing and adjusting the density and the acidity of the copper electrotyping solutions has been prepared, and may be obtained upon request from the Bureau of Standards, Washington, D. C
