The phenomena of life are governed by the laws of gaseous pressure, since all these phenomena take place in solutions. The fundamental laws of biology are those of the distribution of substances in solution, which is regulated by the laws of gaseous pressure, since all these laws are applicable also to osmotic pressure. _Boyle's Law_.--When a gas is compressed its volume is diminished. If the pressure is doubled, the volume is reduced to one-half. The quantity V x P, that is the volume multiplied by the pressure, is constant. _Gay-Lussac's Law._--For a difference of temperature of a degree Centigrade all gases dilate or contract by 1 / 273 of their volume at 0deg Centigrade. {17} _Dalton's Law._--In a gaseous mixture, the total pressure is equal to the sum of the pressures which each gas would exert if it alone filled the whole of the receptacle. _Pressure proportional to Molecular Concentration._--The above laws are completely independent of the chemical nature of the gas, they depend only on the number of gaseous molecules in a given space, _i.e._ on the molecular concentration. If we double the mass of the gas in a given space, we double the number of molecules, and we also double the pressure, whatever the nature of the molecules. We may also double the pressure by compressing the molecules of a gas, or of several gases, into a space half the original size. The molecular concentration of a gas, or of a mixture of gases, is the ratio of the number of molecules to the volume they occupy. The pressure of a gas or of a mixture of gases is proportional to its molecular concentration. This is a better and a shorter way of expressing both Boyle's law and Dalton's law. One gramme-molecule of a gas, whatever its nature, condensed into the volume of 1 litre, has a pressure of 22.35 atmospheres. Similarly one gramme-molecule of a solute, whatever its nature, when dissolved in a litre of water, has the same pressure, viz. 22.35 atmospheres. _Absolute Zero._--According to Gay-Lussac's law, the volume of a gas diminishes by 1 / 273 of its volume at 0deg C. for each degree fall of temperature. Thus if the contraction is the same for all temperatures, the volume would be reduced to zero at -273deg C. This is the absolute zero of temperature. Temperatures measured from this point are called absolute temperatures, and are designated by the symbol T. If _tdeg_ indicates the Centigrade temperature above the freezing point of water, then the absolute temperature is equal to _tdeg_ + 273deg. _The Gaseous Constant._--Consider a mass of gas at 0deg C. under a pressure P_o, with volume V_o. At the absolute temperature T, if the pressure be unaltered, the volume of this gas will be V_oT / 273. Therefore the constant PV, the product of the pressure by the volume, will be represented by P_oV_oT / 273. {18}