Part 108

Where the proportion of protein is relatively large the ratio is called narrow, 1: 4 ... 6. When the proportion of protein is relatively small the ratio is called broad 1: 8 ... 12. In feeding, the nutritive ratio is varied in harmony with the purpose in view, a narrow ratio favoring the development of muscular energy, and a wide one promoting the deposition of fat and the development of heat. These principles guide the scientific farmer in mixing rations for his stock, the work horses receiving a comparatively narrow and the beeves a relatively wide ratio in their food.

=561. Calorimetric Analyses of Foods.=—The general principles of calorimetry have been already noticed. The theoretical and chemical relations of calorimetry have been fully discussed by Berthelot, Thomsen, Ostwald and Muir. In the analyses of foods the values as determined by calculation or combustion are of importance in determining the nutritive relations.

Atwater has presented a résumé of the history and importance of the calorimetric investigations of foods to which the analyst is referred.

In the computation of food values the percentages of proteids, carbohydrates and fats are determined and the required data obtained by applying the factors 4100, 5500 and 9300 calories for one gram of carbohydrates, proteids and fats respectively.

For most purposes the computed values are sufficient, but it is well to check them from time to time by actual combustions in a calorimeter.

=562. Combustion in Oxygen.=—The author made a series of combustions of carbonaceous materials in oxygen at the laboratory of Purdue University in 1877, the ignition being secured by a platinum wire rendered incandescent by the electric current. The data obtained were unsatisfactory on account of the crudeness of the apparatus. The discovery of the process of burning the samples in oxygen at a high pressure has made it possible to get expressions of thermal data which while not yet perfect, possess a working degree of accuracy. The best form of bomb calorimeter heretofore employed is that of Hempel, as modified by Atwater and Woods.

A section of this calorimeter, with all the parts in place, is shown in Fig. 120.

In the figure the steel cylinder A, about 12.5 centimeters deep and 6.2 in diameter, represents the chamber in which the combustion takes place. Its walls are about half a centimeter thick and it weighs about three kilograms. It is closed, when all the parts are ready and the sample in place, by the collar C, which is secured gas tight by means of a powerful spanner. The cover is provided with a neck D carrying a screw E and a valve screw F. In the neck D, where the bottom of the cylinder screw E rests, is a shoulder fitted with a lead washer. Through G the oxygen used for combustion is introduced. The upper edge of the cylinder A is beveled and fits into a groove in the cover B, carrying a soft metal washer. To facilitate the screwing on of the cover, ball bearings KK, made of hard steel, are introduced between the collar and the cover. The platinum wires H and I support the platinum crucible holding the combustible bodies which are ignited by raising the spiral iron wire connecting them to the temperature of fusion by an electric current. The combustion apparatus when charged is immersed in a metal cylinder M, containing water and resting on small cylinders of cork. The water is stirred by the apparatus LL. The cylinder M is contained in two large concentric cylinders, N, O, made of non-conducting materials and covered with disks of hard rubber. The space between O and N may be filled with water. The temperature is measured by the thermometer P, graduated to hundredths of a degree and the reading is best accomplished by means of a cathetometer.

=563. The Williams Calorimeter.=—The calorimeter bomb has been improved by Williams by making it of aluminum bronze of a spheroidal shape. The interior of the bomb is plated with gold. By an ingenious arrangement of contacts the firing is secured by means of a permanently insulated electrode fixed in the side of the bomb. The calorimetric water, as well as that in the insulating vessel, is stirred by means of an electrical screw so regulated as to produce no appreciable degree of heat mechanically. The combustion is started by fusing a fine platinum wire of definite length and thickness by means of an electric current. The heat value of this fusion is determined and the calories produced deducted from the total calories of the combustion. The valve admitting the oxygen is sealed automatically on breaking connection with the oxygen cylinder. The effluent gases, at the end of the combustion, may be withdrawn through an alkaline solution and any nitric acid therein thus be fixed and determined.

=564. Manipulation and Calculation.=—The material to be burned is conveniently prepared by pressing it into tablets. The oxygen is supplied from cylinders, of which two should be used, one at a pressure of more than twenty atmospheres. By this arrangement a pump is not required.

In practical use, a known weight of the substance to be burned is placed in the platinum capsule, the cover of the bomb screwed on, after all adjustments have been made, and the apparatus immersed in the water contained in M, which should be about 2° below room temperature. All the covers are placed in position and the temperature, of the water in M begins to rise. Readings of the thermometer are taken at intervals of about one minute for six minutes, at which time the temperature of the bomb and calorimetric water may be regarded as sensibly the same. The electric current is turned on, the iron wire at once melts, ignites the substance and the combustion rapidly takes place. In the case of bodies which do not burn readily Atwater adds to them some naphthalene, the thermal value of which is previously determined. The calories due to the combustion of the added naphthalene are deducted from the total calories obtained.

The temperature of the water in M rises rapidly at first, and readings are made at intervals of one minute for five minutes, and then again after ten minutes. The first of the initial readings, the one at the moment of turning on the current, and the last one mentioned above are the data from which the correction, made necessary by the influence of the temperature of the room, is calculated by the following formulas.

The preliminary readings of the thermometer at one minute intervals are represented by t₁, t₂, t₃ ... tₙ₁. The last observation tₙ₁ is taken as the beginning temperature of the combustion and is represented in the formulas for calculations by Θ₁. The readings after combustion are also made at intervals of one minute, and are designated by Θ₂, Θ₃ ... Θₙ. The readings are continued until there is no observed change between the last two. Generally this is secured by five or six readings.

The third period of observations begins with the last reading Θₙ, which in the next series is represented by tʹ₁, tʹ₂ ... tʹₙ₂.

In order to make the formulas less cumbersome let

tₙ₁ - t₁ ------------ = v, n₁ - 1

tʹₙ₁ - tʹ₁ ------------- = vʹ, n₂ - 1

t₁ + t₂ + t₃ ... tₙ₁ --------------------------- = t, n₁

tʹ₁ + tʹ₂ + tʹ₃ ... tʹₙ₂ and -------------------------------- = tʹ. n₂

The correction to be made to the difference between Θₙ - Θ₁ for the influence of the outside temperature is determined by the formula of Regnault-Pfaundler, which is as follows:

v - vʹ ⁿ⁻¹ Θₙ + Θ₁ ∑ Δt = -------------- ( ∑ Θr + -------- - nt) - (n - 1)v, tʹ - t ₁ 2

ⁿ⁻¹ in which ∑ Θr ₁

is calculated from the observation of the thermometer Θ₁, Θ₂ etc., made immediately after the combustion. It is equal to the sum of observations Θ₁, Θ₂ etc., increased by an arbitrary factor equivalent to (Θ₂ - Θ₁)/9, which is made necessary by reason of the irregularity of the temperature increase during the first minute after combustion, the mean temperature during that minute being somewhat higher than the mean of the temperatures at the commencement and end of that time.

The quantity of heat formed by the combustion of the iron wire used for igniting the sample is to be deducted from the total heat produced. This correction may be determined once for all, the weight of the iron wire used being noted and that of any unburned portion being ascertained after the combustion.

Ten milligrams of iron, on complete combustion, will give sixteen calories.

In the combustion of substances containing nitrogen, or in case the free nitrogen of the air be not wholly expelled from the apparatus before the burning, nitric acid is formed which is dissolved by the water produced.

The heat produced by the solution of nitric acid in water is 14.3 calories per gram molecule. The quantity of nitric acid formed is determined by titration and a corresponding reduction made in the total calculated calories.

In the titration of nitric acid it is advisable to make use of an alkaline solution, of which one liter is equivalent to 4.406 grams of nitric acid. One cubic centimeter of the reagent is equivalent to a quantity of nitric acid represented by one calorie.