The vacuum of space may be emptier than the best vacuum that we can make with current technology, but it is not completely empty; every cubic centimeter of the emptiest parts of space contains about one atom. this gas, which exists everywhere between the stars, is part of the interstellar medium (ism). based on what you have learned about light from previous chapters, how would we determine what this gas in the interstellar medium is made of?

Ageologist is working in an area with exposed sedimentary rocks and finds a well exposed sequence of stratified rocks. as he works through a thick sequence of limestone, he sees abundant large clam fossils that produce an interlocking texture suggesting a reef, but within the limestone section these clams disappear and the overlying limestone is made up primarily of fragments of corals. he now moves 50 km away and finds limestones interbedded with shales, and the limestones contain the same clam fossils he had seen previously 50 km away. as he continues through this section, the rocks become entirely limestone, and the clams disappear again with coral fossils above forming a reef structure. he concludes