A phenomenon can not always be separated from the accessory phenomena which concur with it in producing the final result. Necessity of corrections to render complex results comparable _inter se_. Density of solids when regard is had to the temperature and weight of the gases displaced by them. Precautions to be attended to in the experiments. Empirical formulæ for the density of liquids. Maximum density of water. The temperature corresponding to the maximum must be determined graphically, or by interpolation. Corrections for measures of capacity, for barometric measures. The uncertainty of the corrections can not, in any considerable degree, affect the densities of solids and liquids. Density of gases. Biot and Arago’s experiments. Special difficulties of the question. The uncertainty of the corrections may sensibly affect the results. Regnault’s method. The same method may be applied to the determination of the co-efficient of dilatation for gases. Density of vapors. Definition founded on the hypothetical application of the same laws to gases and vapors. Formulæ. Experimental method of Gay-Lussac and of Dumas. Corrections. Comparison of the two methods. Necessity of conducting the experiments at a distance from the saturation point. Latour’s experiments. Relations between the weight and volume of a gas, and its temperatures; between the weight and volume of a gas mixed with vapors, and its temperature. Various problems. Hygrometry. Chemical hygrometry. Hygrometry by the dew-point. Psychrometry. PROPAGATION OF HEAT. LESSONS 17-18. _Propagation at a Distance._ Rapid propagation of heat at a distance, in vacuum, in gases, in certain liquid or solid mediums. Experiments which establish this. Rays of heat. Velocity of propagation. Intensity of heat received at a distance. Intensity of heat received or emitted obliquely. Emitting power, power of absorption, reflection, diffusion. The emitting and absorbing power are expressible by the same number in terms of their proper units respectively. Analysis of calorific radiations by absorption. Different effects of deathermanous or thermochroic medium. Different influences of increasing thicknesses of the combination of different mediums. Radiations proceeding from different sources, various effects of different mediums on these radiations. The calorific radiations emanating from different sources, have all the characters of differently colored heterogeneous rays of light. THEORY OF RADIATION AND OF THE DYNAMICAL EQUILIBRIUM OF TEMPERATURES. APPARENT REFLECTION OF COLD. LESSON 19. _Law of Cooling._ Definition of the rate of cooling. Many causes may conspire in the cooling of a body.