The fifth tube we will examine for common salt, which is nearly always present in water. This test is done by dropping in some nitrate of silver solution, which produces a white precipitate, caused by the silver combining with the chlorine, which is the principal element in common salt. This is an excessively fine test, and the nitrate of silver will detect an incredibly small trace of common salt in water. The worst effect that common salt has in water, is to cause priming. Where it is present to any great extent, the blow-off cock should be used frequently on the road; and a surface-cock is quite an aid in keeping the boiler in order. Water that is taken from wells or streams about cities, and found to give strong chlorine re-actions, is nearly always contaminated with sewage.
LEARNING THE MANIPULATION OF TESTS.
Practice in making superficial qualitative tests of water, produces skill in reading the meaning of the various chemical re-actions. This skill can be rapidly developed by practice on prepared specimens. Water for experiments on carbonate of lime can be prepared by dissolving calcite crystals or marble dust in hydrochloric acid, or by mixing chalk with clean rain-water, and filtering it till free from turbidity. While the chalk is mixed with the water, and unfiltered, the specimen will be made stronger by blowing air from the lungs through a glass tube into the water. A preparation for testing sulphate of lime may be made by dissolving some gypsum in distilled water. Fluid magnesia, dropped into pure water, will provide carbonate of magnesia specimens; and Epsom salts will give the magnesia re-action, with the addition that it will indicate sulphuric acid under the chloride of barium test. A grain of salt no larger than a pin-head dropped into a pint of distilled water, will give a distinct chlorine test when nitrate of silver solution is added. Stronger, or even weaker, salt solution can be used by the experimenter while he is working into practice. A small piece of sulphate of iron, dissolved in water, will provide a test for iron.
MAKING QUALITATIVE TESTS.
While pursuing the apprentice practice of tests with these solutions, care must be taken to have the test tubes perfectly clean. Go over the tests in something like the following order. The carbonate of lime test is to be made first. Four test tubes will be used. In the first we put distilled water alone; in the second we put ten drops of the chalk solution; in the third we put twenty drops of the solution; in the fourth we put thirty drops, then add distilled water till all the test tubes are about two-thirds filled. Drop into each tube about the same quantity of oxalate of ammonia solution, and the degree of turbidity will help to indicate the hardness of each specimen. The first tube, containing pure water, will give no re-action if the chemicals are pure.
This process should be extended to all the other solutions, and will be found very helpful. In carrying out experiments of this kind, much assistance will be obtained from having a work like Stuckhardt’s Chemistry at hand for reference.
THE SOAP-TEST FOR HARDNESS.
Most people are aware that hard water has a peculiar effect upon soap, making it curdle instead of lather when used for washing purposes. This peculiarity was made use of some years ago by Dr. Clark of Aberdeen, Scotland, in devising a test for the hardness of water, in which the quantity of a standard soap solution needed to produce a permanent lather on water, indicated the degree of hardness of the water. A modification of the Clark process can be used very conveniently by master mechanics in making superficial tests of the hardness of water, and the plan has the advantage of being easily applied. With a test tube and a small bottle of soap-solution, the investigator is ready at any time or place to make tests; and a few minutes spent over each specimen will give him an idea of the value of the water for boiler purposes.
The original Clark soap-test was made with a soap-solution of known strength, of which a certain measured quantity was required to produce a permanent lather on a gallon of water containing a given quantity of carbonate of lime. The degrees of hardness of other specimens were computed according to the quantity of the soap-solution required to produce a permanent lather. Preparing soap-solution of a certain strength, and water of a certain hardness, for the purpose of indicating a point for beginning the computations, is a tedious operation; and the soap-test can be used in a much simpler way. While making numerous tests of water on the Burlinglington, Cedar Rapids, and Northern Railway some years ago, to ascertain the condition of water tanks at different seasons of the year, and in examining the supply of proposed water stations on extensions of the road, I used the following modification of the Clark process:—