A collection of online activities emphasizes the design and interpretation of experiments. C hemistry concepts are abstract and can be diffi cult to attach to real-world experiences. For this reason, highschool and college chemistry courses focus on a concrete set of problem types that have become canonized in textbooks and standard exams. These problem types emphasize development of the core notational and computational tools of chemistry. Even though these tools may form the underlying procedural knowledge base from which the "real stuff " can be approached, when taught out of contexts that show their utility or that draw connections to core ideas of science, they can appear as a disconnected bag of tricks (1).The ChemCollective (www.chemcollective. org) is a digital library of online activities for general chemistry instruction that engages students in more authentic problem-solving activities than those found in most textbooks. Our goal is to create activities that allow students to use their chemistry knowledge in ways that resemble the activities of practicing chemists. Our guiding hypothesis is that better conceptual understanding is obtained if algebraic computations are complemented with design and interpretation of experiments. This is achieved through the ChemCollective "Virtual Lab," which allows students to design and carry out their own experiments while experiencing representations of chemistry that go beyond what is possible in a physical laboratory. The goal is not to replace, or even to emulate, the physical laboratory, but to supplement textbook problem-solving by connecting abstract concepts to experiments and real-world applications. We believe that such authentic activities may improve learning and may better help to bring the essence of science into the introductory chemistry classroom.In the virtual lab (see the fi gure, right), the panel on the left is a customizable stockroom of chemical reagents, which may include common reagents or fi ctional materials that have properties specifi ed by the instructor. The middle work space provides an area for performing experiments. The right panel provides multiple representations of the contents of the selected solution, including the temperature and pH, and a list of chemical species with amounts shown as moles, grams, or molar concentrations. These quantities are the players in the computational procedures of the course, and so this panel provides an explicit link between the paper-and-pencil calculations of the traditional course and the chemical experiments the student performs on the workbench.The virtual lab supports new forms of problem-solving. Consider how the concept of limiting reagents in a reaction is usually taught. A student's practice with this concept typically centers around learning a standard computational procedure for predicting the fi nal amount of chemical reagents, given the initial amounts. Our "unknown reaction" virtual lab activity provides a different mode of practice. Students are given four unknown chemicals (A, B, C...

Shemakha Astrophysical ObservatoryWith 12 figures (Received 1979 .\ugust I ) Ten-colour photoelectric observations are presented for twelve Ap-stars. Improved ephemeris for seven of them is given. Phase relations between the light curves and line intensity variations are discussed. The problem of thc electromagnetic flux conctancy of IOTA Cas is approached from a qualitative point of view.

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1: s wird cine Mcthotlc zur I'crioclciisuclic in unifaiigreiclicn hIcUrcihen h*schricbcri. I h s Vcrfiilircii k r u l i t , \vie die Mctliotle \on LAFLER untl KISMAS, auf clcr Ucrechnung clcr Strcuutig. Da das Verfahren abcr nicht tler Sortierung der MeUwcrte nacli tlcr I'hasc bedarf, lasscn sich I'eriodogranimc wcscntlich schncllcr krcchnen. Bin I~OKTH.4N-l'rogranim wird angegcben..\ method for the search of periods in large series of observational data is discribcd. Like that proposed by LAFLER and lirs-M A S our procedure is bascd on the computation of the variance, however, the speed of computation of periodogranims is coiisidcrably higher since w c tlo not need to size the valucs with regard to the phasc. A 1WRTRAN-programm is given.