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November 6, 1885.J THE PHOTOGRAPHIC NEWS. 709 vith a piece of litmus paper. When reached, add a few drops of the chro- It may be useful to mention here, that there is now in the market a comparatively pure form of carbonate of soda, containing only 1 molecule of water, NagCO:H,O, known by the name of “ crystal carbonate,” which will be found better for photographic purposes than the ordinary com mercial anticle. the neutral point is mate of potash solution, and run in standard silver from the burette till the same red tint is reached as in pre- —say 70 grs. Dissolve in about 4 ounces distilled water, and neutralize by adding pure nitric acid gradually, and boiling, testing from time to time with a chromate will not be decomposed, but will impart a red colorzration to the precipitate, and this marks the point where we have added sufficient silver. The amount of solution used is now read off from the burette, and the same operations repeated with the other quantities of salt solution, care being taken in each case to stop the addition of silver when the precipitate has reached the same tint, as with the preceding quantity of chloride of sodium. It is obvious that as we dissolved 58’5 grs. of chloride of sodium in 500 c.c., that each 25 cc. of this solution contains 2'925 grs. Suppose for the first test we used 40 cc., the second 401 cc., and the third 39'9 cc. Taking the average of the three tests we find that 40 cc. of our solution are equal to 2'925 grs. of salt, or 800 cc. = 58-5 grs., an equiva lent of sodium chloride. Therefore, if 800 cc. of the solu tion be taken, and made up with distilled water to 1,090 cc., we shall obtain the standard solution we require. We again test this with the salt solution to make sure that we have conducted all the operations with accuracy; 50 cc. of the silver solution should be equal to 25 cc. of the salt solution. Having thus completed the standard silver solution, we are in a position to test the sample quantitatively for chlo ride in a very short time, and with very little trouble. For example, suppose we require to find the chloride of sodium in a sample of carbonate of soda. We know by the quali tative test whether to expect much or little chloride in a sample, and weigh out a quantity of the substance, which will require somewhere about the same quantity of silver solution as was used in making up the standard solution . Suppose, in the sample, we expect to find under 6 per cent, of chloride of sodium, weigh out between 50 and 100 grs. METEOROLOGY FOR PHOTOGRAPHERS. BY J. VINCENT ELSDEN, B.SC. (LOND.), F.C.S. Chapter VI.—Atmospheric Moisture {continued.')—Dew, Mist, and Cloud—Forms of Clouds. In the last chapter it was stated that the amount of aqueous vapour which a given quantity of air can retain depends chiefly upon its temperature, and that when cooled below the point of saturation, seme of the invisible vapour assumes the visible ’form of cloud, mist, dew, or rain. This fact may be readily proved by the simple experiment of ex hausting the air from the receiver of an air pump. In this way an artificial cloud may be formed, owing to the cooling of the rarefied air by its sudden expansion. A. similar expansion takes place when a current of air ascends into the upper regions of the atmosphere ; and if it is cooled sufficiently, there will he a similar condensation of the aqueous vapour which it contains. Chilling of air may also result from its contact with cold mountain tops, or from the meeting of warm and cold currents in the atmosphere. Now this condensation of aqueous vapour is a true change from an invisible gas into visible particles of water, and these particles may either remain suspended in the air as cloud, mist, or fog; or they may be deposited upon the surface of the earth as dew, hoar-frost, or rain. Let us consider, first, the formation of dew. In the evening, the heat derived from the sun no longer balances that which is radiated by the earth into space. The sur face of the earth, therefore, grows cooler in proportion to the rapidity with which radiation proceeds. The air in contact with the cold earth is cooled also, and, if the limit of saturation is passed—that is to say, if the temperature falls below that which corresponds to the maximum vapour-tension of the vapour present in the air—dew will be deposited ; and if this temperature is below freezing point, the dew will take the form of hoar-frost. Hence dew will be most plentiful on the coldest objects—that is, on the best radiators—and any cause which tends to check radiation of heat from the earth, will tend also to prevent the formation of dew. Fogs, mist, and clouds are formed by a similar process, but the particles of water remain suspended in the atmo sphere. Fogs are generally caused by the mixture of masses of air which differ so much in temperature that the resulting temperature is too low to retain in the form of vapour the moisture which is present. In this case it is thought that foreign bodies, such as dust and soot, play an important part by acting as a nucleus of condensation ; and Mr. Aitken has attempted to show that the formation of mist is impossible in the absence of any such particles. The manner in which the particles of water constituting mist and fog remain suspended in the atmosphere is a question which cannot be looked upon as decided. Some maintain that these particles are hollow vesicles ; but it seems more probable that their buoyancy is simply owing , to their extremely minute subdivision, so that Tyndall has applied the term “water-dust” to express the nature of mist and cloud. Let us now take a few of the more familiar examples of . the formation of fogs and mists. Over running streams i there is often a bank of fog, whether the water is warmer or colder than the overlying air. In time of frost, the ■ water is usually warmer than the air, and consequently i gives off more vapour than the cold air can retain. When the air is warmer than the water, fog may still be pro duced by its temperature being lowered below the point of saturation by contact with the water In a similar way paring the standard. Suppose 70 cc. are required. Then the equation- 100 : 5'85 = 70 : 3'595 gives the quantity of chloride of sodium in the solution ; and the following equation gives the percentage of chlo ride of sodium in the sample :— 70:3-595 = 100:5'137 When estimating small percentages of a. substance, it is best to use comparatively large quantities of the sample, for if this is done small losses and inaccuracy from other causes do not make so much difference as if smaller quantities of the material is used. It is for a similar reason that we Weighed out the chloride of sodium used in testing the standard silver, and dissolved it in a large quantity of water. Besides, in this way we save the time and trouble of weighing out separately several small quantities of the salt. This method may be used for estimating the chlorine in any substance containing this body in the form of chloride, provided it contains no other compound which will precipi tate silver in a neutral solution, such, for instance, as bro mides and iodide. Bearing in mind that 100 c.c. of the silver solution are equivalent to 31565 grs. of chlorine in the form of chloride, it is a simple matter to find the weight of the chloride we wish to estimate. Suppose, for instance, we wish to find the weight of calcium chloride equal to 50 c.c. of the silver solution. Calcium chloride is represented by the formula CaCla, of which the equivalent weight is (40-35*5+355) 5111. There being two atoms of chlorine in calcium chloride, one equivalent of it requires twice as much silver solution as an equivalent of sodium chloride. Therefore our proportion will be :— 100 : -11.= c.c. of silver solution : x -X1V
