powders are the result of these investigations. Gluten is not changed chemically by the action of the yeast or of the carbon dioxide, but it is physically changed-the escape of the gases stretching it out into fibres. Gluten, like other proteids, hardens when heated. Baking thus makes the porous condition of the dough permanent. MAKING BAKING POWDER Several students have sent me recipes they like to use for making baking powder. The claim is made that these cost rather less than the kinds that can be bought, and also that they are much more effective. Here is one: 1⁄2 lb. cream of tartar. 14 lb. cooking soda (bicarbonate of soda). 1 lb. corn starch. Sift The best quality of each must be bought. them together at least a dozen times, the last time into baking powder boxes. Be careful to seal up all cracks by pasting over them paper strips. About one half as much of this is required as for the average powder sold. These proportions would probably give a slight excess of acid. We might combine 24 parts of the acid salt with one part of soda if our salts are chemically pure. The corn starch is added to keep the soda and acid salt from forming quite such an intimate mixture. The two salts in contact would very slowly combine, and the baking powder thus lose its strength. DISTILLATION A few more words might be said on the subject of distillation. I am sometimes asked to explain more fully the term "destructive distillation." When a complex substance like wood or coal is heated some of its ingredients are made volatile at the high temperature, and so escape as gases. The wood itself is broken up into simpler substances. It is plain that in this process the original substance is lost as such, new substances taking its place, and we therefore speak of the process as destructive distillation. When water containing various salts or gases in solution is heated, the gases will be given off as the temperature rises. At the boiling point, the water itself will begin to pass off as vapor. The salts will not vaporize unless much more strongly heated. If the steam be collected and cooled, it will condense to form pure water. This in an illustration of simple distillation. If a mixture of alcohol and water be heated some of the alcohol will vaporize before the water. It may in this way be separated from the water, and this process is called fractional distillation. This is the principle employed in the manufacture of whiskey, etc. COMPOSITION OF GAS The complex nature of coal gas is shown by the following table, which represents an average sample: Of these, the hydro-carbons, carbon monoxide, CH4 and hydrogen are combustible. Coals always contain more or less sulphur, v. hich is a great trouble to the gas manufacturer. It frequently happens that some of it gets into the gas. If such gas escapes, the sulphur compounds unite with the silverware, giving is a coating of dark sulphide of silver. If silver tarnishes quickly, it is an indication of a leak of gas or sewer gas. It is estimated that a ton of coal should yield 10,000 feet of gas, 1,400 lbs. of coke (35 bushels), 12 gallons of tar, 4 lbs. of ammonia. More than six hundred products are obtained from the coal tar. The nature and uses of these products would form an interesting topic for futher study. The composition of water gas is somewhat as follows: Notice that this gas contains less methane and hydrogen (which are combustible), and their place is taken by carbon monoxide, which, although combustible, is very poisonous. There is some carbon monoxide in ordinary illuminating gas but not nearly so much. The water gas has a strong odor from the hydro-carbons (crude gasoline) added to make it. luminous, but comparatively little of it in the air is likely to produce very injurious effects upon living things, plants and animals alike. It is the most poisonous substance that comes into the house. It is estimated that about fourteen per cent of the gas manufactured escapes into the earth through leaky gas mains. In passing through the soil the odorous part of water gas may be strained out, so that it becomes odorless. Whole families have been poisoned from deodorized water gas leaking into the house by way of the cellar. This emphasizes the importance of having a perfectly tight cellar, with cemented walls and floor, and the importance of ventilating the cellar, for the cellar air finds its way to the rooms above. Natural gas contains practically no carbon monoxide. SPONTANEOUS COMBUSTION We often hear of fires apparently "starting themselves." Such cases are due to accumulation of heat produced by slow oxidation. If a pile of oily rags, cotton waste, etc., be allowed to stand for a time, the oily matter will begin to combine slowly with oxygen. This may occur in the inner part of the heap, and the outer layers retain the heat until, perhaps, the kindling point of some of the inflammable oils is reached, when the whole mass will burst into flame. This is much more likely to happen with linseed oil and certain other vegetable "drying oils," as they unite readily with oxygen, and so become hard and varnishlike. The mineral oils (paraffine oil) do not combine with oxygen at ordinary temperatures, and probably will not cause spontaneous combustion. Still, all oily cloths should be burned or disposed of in some safe fashion. CONSERVATION OF ENERGY An interesting and important principle, explained on page 23 of Part I, and again on page 110 of Part III, is Conservatism. This principle has been established by countless experiments, but it is not |