We must conclude this chapter by making a few remarks on the important matter of the regulation of light and temperature. Direct sunlight should always be avoided, except for short and occasional intervals, not only because it is liable to raise the temperature to a higher degree than is suitable for the inmates of the aquarium, but also because an excess of light and heat tends to produce a rapid decomposition of organic matter, and a consequent putrid condition of the water, and this dangerous state is most likely to occur when both light and temperature are high at the same time. The water should always be cold; and as it is not always easy to estimate the temperature, even approximately, by the sensation produced on immersing the fingers, it is a good plan to have a small thermometer always at hand, or placed permanently in the aquarium. In the summer time the water should be kept down to fifty-five degrees or lower, and in winter should never be allowed to cool much below forty. There may be some difficulty in maintaining a temperature sufficiently low in summer, but a small piece of ice thrown in occasionally to replace the loss due to evaporation, especially on very hot days, will help to keep it down. CHAPTER V _THE PRESERVATION OF MARINE OBJECTS_ The sea-side naturalist, in the course of his ramblings and searchings on the coast, will certainly come across many objects, dead or alive, that he will desire to set aside for future study or identification in his leisure moments at home. Some of these will be required for temporary purposes only, while, most probably, a large proportion will be retained permanently for the establishment of a private museum, that shall serve not only as a pleasant reminder of the many enjoyable hours spent on the shore, but also as a means of reference for the study of the classification of natural objects and of their distribution and habitats. We will first deal with those specimens that are required for temporary purposes only--those of which the collector desires to study the general characters, as well as, perhaps, something of the internal structure; but before doing so we cannot refrain from impressing on the reader the advisability of learning as much as possible of the external features and mode of growth of the different living creatures while still alive, for it must be remembered that it is impossible to preserve many of them without more or less destruction of their natural colouring and distortion of their characteristic forms.