CHAPTER I APPARATUS, REAGENTS, WEIGHTS AND MEASURES THE methods of analysis of, and modes of examining the substances of which the body is composed vary a good deal from those ordinarily employed in chemistry. An examination of a living tissue by purely chemical means is well-nigh impossible, as the reagents used will be almost certain to destroy the life of the tissue in question. Hence some of the methods adopted by the physiologist differ markedly from those of the chemist: for instance, he may experiment upon himself or upon animals, giving certain foods or drugs by the mouth, and examining the result in the alimentary canal or their effect on the urine and other excretions; again, he may attempt to imitate after death the conditions which obtain during life, and so conduct experiments upon ferment action, the changes in cells, and so on. The physiologist has to deal very largely with an important class of substances known as albuminous or proteid ; these require certain special methods of investigation which are but rarely used in the work of ordinary chemistry; and then the physiologist avails himself of certain physical apparatus, such as the spectroscope, polariscope, &c., which give material help in the elucidation of chemical problems. The reader must consult some special book on analytical chemistry for full details respecting analytical methods. The present chapters (Part I) form a mere sketch of the chief operations performed in chemical investigation, those specially available for physiological work being dwelt on rather more fully. WEIGHTS AND MEASURES The weights and measures usually employed in science are those of the metric system; but, as the practical physician still uses very largely English grains and ounces, we give here both systems. The scruple = 20 grains = 1.296 grammes, and the drachm = 60 grains = 3.888 grammes are retained in use, but neither is an aliquot part of the ounce; though for practical purposes an ounce is considered to consist of 8 drachms. The standard of length is a metre; subdivisions and multiples of which, with the prefixes milli-, centi-, and deci- on the one hand, and deca-, hecta-, and kiloon the other, have the same relation to the metre as the subdivisions and multiples of the gramme, in the table just given, have to the gramme; thus: 1 millimetre = 0·001 metre = 0·03937 inch In the metric system the measures of capacity are intimately connected with the measures of length; we thus have cubic millimetres, cubic centimetres, and so forth. The standard of capacity is the litre, which is equal to 1,000 cubic centimetres; and each cubic centimetre is the volume of 1 gramme of distilled water at 4° C. 1 cubic centimetre (generally written c.c.) = 16-931 minims 1 litre = 1,000 cubic centimetres = 1 pint 15 oz. 2 drs. 11 min. = 35·2154 fluid ounces 1 cubic inch = 16:386 c.c. 14° C. is the temperature at which water has the greatest density. For practical purposes, measures are more often constructed so that a cubic centimetre holds a gramme of water at 16° C., the usual temperature of a room. The true c.c. contains only 0·999 gramme at 16° C. THERMOMETRIC SCALES The scale most frequently used in this country is the Fahrenheit scale; in which the freezing point of water is 32°, and the boiling point 212°. On the Continent the Réaumur scale is largely employed; in which the freezing point is 0° C., the boiling point 80°. In scientific work the centigrade scale has almost completely taken the place of these; in this system the freezing point is 0°, and the boiling point 100°. To convert degrees Fahrenheit into degrees centigrade, subtract 32 and multiply by 3, or C=(F-32). Conversely, degrees centigrade may be converted into degrees Fahrenheit by the following formula : F=C+32. TENSION OF AQUEOUS VAPOUR IN MILLIMETRES OF MERCURY FROM 10° TO 25° C. These numbers are used in correcting measurements of wet gases--for instance, of the nitrogen obtained from urea by the hypobromite method. TABLE OF THE DENSITY OF WATER AT TEMPERATURES BETWEEN 0° AND 30° C. |