This paper reports measurements of the elasticities of various Weimarn gels by the use of an ultrasonic pulse reflection method. Considerations of porosity and composite density of the gel lead, theoretically, to an expression for the bulk modulus which agrees remarkably well with the experimental values. Furthermore, the aging of the gel samples gives ground to conclude that the formation of a gel resembles the separation of a solid phase from a supersaturated solution. The variation of the elastic moduli with temperature has been explained by the postulated structure of these gelatinous substances.